Compounds binding to the bacterial beta ring

ABSTRACT

The present invention relates to compounds which bind to the hydrophobic pocket of the β clamp, i.e., to the surface of the β ring with which said protein interacts with other proteins of the bacterial replication complex during DNA replication. These compounds are derived from the acetylated peptide AcQLDLF (P6) to improve their affinity to their target.

SEQUENCE LISTING SUBMISSION VIA EFS-WEB

A computer readable text file, entitled “045636-5242-SubstituteSequenceListing.txt” created on or about 14 Apr. 2015, with a file size of about 13 kb contains the sequence listing for this application and is hereby incorporated by reference in its entirety.

The present invention relates to bacterial replication. More precisely, the present invention concerns compounds which bind to the hydrophobic pocket of the β clamp, i.e., to the surface of the β ring with which said protein interacts with other proteins involved in DNA metabolism.

In all three domains of life, multicomponents complexes, the so-called replisomes, have evolved to ensure the faithful replication of chromosomal DNA. One central protein of these complexes forms a ring that encircles and slides along the double stranded DNA^(1, 2). A physical interaction between the clamp and the chromosomal replicase confers a high processivity to the enzyme³. In bacteria, the processivity factor, also referred to as the β ring, is a homodimer which results from the head-to-tail association of two monomers, each of them being shaped in three globular sub-domains¹. In eukaryotes and archae, the β homolog factor, PCNA (for Proliferating Cell Nuclear Antigen), is a homotrimer with each monomer organized in two sub-domains^(2, 4).

Beside their role as processivity factors for chromosomal replicases, β and PCNA clamps also participate in various protein-protein interactions. They notably act as landing platforms for factors involved in DNA metabolism and cell cycle regulation⁵, particularly DNA polymerases involved in translesion synthesis^(6, 7), and factors promoting DNA repair^(8, 9, 10). All these factors possess a small conserved peptide sequence, which binds into a hydrophobic pocket located on one side of the ring. Noteworthy, these pockets differ significantly between bacterial rings and PCNA. A bioinformatics analysis performed on putative β ring partners led to define the bacterial consensus binding peptide QL[S/D]LF¹⁰. The absolute requirement of the interacting peptide for β ring partners binding has been further demonstrated biochemically and physiologically^(11, 12, 13, 14). Finally, the interaction between the ring and the interacting peptide of different β binding proteins have been structurally characterized^(15, 16, 17, 18) The peptide binding site is formed by a deep leucine-rich hydrophobic pocket (subsite 1) located between sub-domains two and three of the β monomer and connected via a groove to a second sub-site (subsite 2) located in sub-domain three¹⁷ (FIG. 1C). An additional interaction has also been observed in the case of the polymerase Pol IV, between the little finger domain of the enzyme and the edge of the β ring¹⁶.

The major contribution of the peptide-mediated interaction to a successful DNA replication and ultimately to cell survival, both in prokaryotes and eukaryotes, makes the ring interacting pockets potential targets for the development of new antibacterial or anticancer drugs, respectively. In a recent report, a chemical compound was identified from a library and shown to bind into the leucine rich sub-domain of the E. coli β ring interacting pocket with an affinity of 10⁻⁵ M¹⁸.

In the experimental work described below, a different, structure-based strategy was used to design short peptides with improved affinities for the β interacting pocket. The first step of this approach was to decipher the molecular basis of the interaction of the natural ligand in the binding pocket. Then, using these data, a first peptide (SEQ ID No: 6, P6) was designed, which was then further modified to improve its affinity. Several biophysical and biochemical methods were used to measure the strength of the interaction and to characterize the structure of the most efficient complexes formed. As a result, the binding efficiency of the modified ligand was improved by two orders of magnitude, reaching 10⁻⁸ M range.

Due to their very good affinity for the β interacting pocket, the compounds described in the present text are very promising leads for new antibiotic compounds.

According to a first aspect, the present invention pertains to a compound of formula (I)

wherein

-   -   Gln is glutamine;     -   R is selected in the group consisting of a C₁₋₁₂-alkyl group         optionally substituted by a C₆₋₁₀-aryl group, a C₂₋₁₂-alkenyl         group optionally substituted by a C₆₋₁₀-aryl group, a         C₃₋₆-cycloalkyl group, a C₆₋₁₀-aryl group optionally substituted         by a C₁₋₄-alkyl, and a C₁₋₅-alkyl-(O—CH₂—CH₂)_(t)— group with t         being an integer from 0 to 20 inclusive;     -   R¹ is the side chain of arginine or lysine (n.b.: when n>1, each         R¹ is, independently from each other, the side chain of arginine         or lysine);     -   R² is a —(CH₂)—C₃₋₆-cycloalkyl group optionally substituted by a         halogen and/or by a group selected amongst —NH₂, —NH—CO—R^(a),         —CO₂H, —NHR^(a) and —NR^(a)R^(b), wherein R^(a) and R^(b) are         independently a C₁₋₄-alkyl group;     -   R³ is selected in the group consisting of a C₁₋₈-alkyl group,         the side chain of arginine or lysine, —(CH₂)_(q)—CO₂R^(7a),         —(CH₂)_(q)—CO—NHR^(7b), —CH₂OR⁸ and — (CH₂)_(q)NHR⁹, wherein         -   q is 1, 2, 3 or 4,         -   R^(7a) is a hydrogen atom, a C₁₋₈-alkyl group, a             C₄₋₁₂-alkylene group forming together with R⁶ a lactone or a             polyether ring, or a C₄₋₁₂-alkenylene, forming together with             R⁶ a lactone or a polyether ring,         -   R^(7b) is a hydrogen atom, a C₁₋₈-alkyl group, or             —(CH₂)_(q′)—NH— with q′ being an integer between 2 and 8             inclusive and forming together with R⁶ a lactam,         -   R⁸ is a hydrogen atom, a C₁₋₈-alkyl group, a C₄₋₁₂-alkylene             group forming together with R⁶ a lactone or a polyether             ring, or a C₄₋₁₂-alkenylene, forming together with R⁶ a             lactone or a polyether ring,         -   R⁹ is a hydrogen atom, or R⁹ together with R⁶ form a lactam;     -   R⁴ is a C₁₋₈-alkyl group optionally substituted by a         C₃₋₆-cycloalkyl group, or a halogen-C₁₋₄-alkyl group;     -   R⁵ is selected in the group consisting of a         —(CH₂)—C₃₋₆-cycloalkyl group; —(CH₂—CH₂)—C₃₋₆-cycloalkyl group;         a —(CH₂)—C₆₋₁₀-aryl group optionally substituted by a halogen, a         C₁₋₂ alkyl group and/or a C₁₋₂ alkoxy group; a         —(CH₂—CH₂)—C₆₋₁₀-aryl group optionally substituted by a halogen,         a C₁₋₂ alkyl group and/or a C₁₋₂ alkoxy group; a         —(CH₂)—C₅₋₁₀-heteroaryl group optionally substituted by a         halogen and/or a C₁₋₂ alkyl group; a —(CH₂—CH₂)—C₅₋₁₀-heteroaryl         group optionally substituted by a halogen and/or a C₁₋₂ alkyl         group;     -   R⁶ is —CO₂H, —CO₂R¹⁰, —CO—NH₂, —CO—NHR¹⁰, —OR¹⁰ when r is 1 or         2, —NH—CO—NHR¹⁰ when r is 1 or 2, or R⁶ is —CO—, —CO—O— or —O—         and forms a lactam, a lactone, or a polyether ring with R^(7a),         R^(7b), R⁸ or R⁹; wherein         -   R¹⁰ is a C₁₋₈-alkyl group optionally substituted by a             C₆₋₁₀-aryl group; a C₃₋₆-cycloalkyl group; a C₆₋₁₀-aryl             group optionally substituted by a halogen, a C₁₋₂-alkyl             group and/or a C₁₋₂-alkoxy group;     -   m is 0 or 1;     -   n is an integer from 0 to 9 inclusive;     -   p is an integer from 0 to 10 inclusive;     -   r is 0, 1 or 2.

In the above formula (I), the peptide linkages (—CO—NH—) can be replaced or modified to obtain synthetic pseudopeptides or peptidomimetics in which the peptide bond is modified, especially to become more resistant to proteolysis, provided the immunogenicity of and the toxicity of the molecule is not increased by this modification, and providing the pseudopeptide retains its affinity for the β interacting pocket.

The following definitions are set forth to illustrate and define the meaning and scope of the various terms used to describe the invention herein. The term “C₁₋₁₂-alkyl” refers to a branched or straight-chain monovalent saturated aliphatic hydrocarbon group of 1 to 12 (inclusive) carbon atoms. Similarly, the terms: “C₁₋₈-alkyl”, “C₁₋₅-alkyl”, “C₁₋₄-alkyl”, “C₁₋₂-alkyl” and the like refer to branched or straight-chain monovalent saturated aliphatic hydrocarbon groups of, respectively, 1 to 8 (inclusive), 1 to 5 (inclusive), 1 to 4 (inclusive), 1 to 2 carbon atoms. This term is further exemplified by groups as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecanyl and their branched isomers. The “alkyl” group can optionally be mono-, di-, tri- or multiply-substituted by a halogen and/or a C₆₋₁₀ aryl group, as defined below.

The term “C₁₋₈-alkyl-(O—CH₂—CH₂)_(t)-” refers to a —(O—CH₂—CH₂)_(t)— substituted C₁₋₈-alkyl group wherein the alkyl group is as defined above and t is an integer from 0 to 20 (inclusive), preferably 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10. Preferred —(O—CH₂—CH₂)_(t)— substituted alkyl group is a C₁₋₅-alkyl-(O—CH₂—CH₂)_(t)— group with t and alkyl as defined above.

The term “C₂₋₁₂-alkenyl” refers to a branched or straight-chain monovalent unsaturated aliphatic hydrocarbon group having one or more carbon double bonds, of 2 to 12 (inclusive) carbon atoms, preferably 2 to 8 (inclusive) carbon atoms, more preferably 2 to 4 (inclusive) carbon atoms. This term is further exemplified by groups as vinyl, propylenyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl, dodecenyl and their straight-chain and branched and stereo isomers. The “alkenyl” group can optionally be mono-, di-, tri- or multiply-substituted by a halogen and/or a C₆₋₁₀-aryl group, as defined below.

The term “C₁₋₁₂-alkylene” refers to a divalent C₁₋₁₂-alkyle with alkyl as defined above. Similarly, terms such as “C₄₋₁₂-alkylene” or “C₄₋₈-alkylene” and the like, refer to divalent C₄₋₁₂-alkyl or divalent C₄₋₈-alkyle group where alkyl is defined above. Examples of alkylene groups are —(CH₂)—, —(CH₂)₂—, —(CH₂)₃—, —(CH₂)₄—, —(CH₂)₅—, —(CH₂)₆—, —(CH₂)₇—, —(CH₂)₈—, —(CH₂)₉—, —(CH₂)₁₀—, —(CH₂)₁₁—, —(CH₂)₁₂—.

The term “C₄₋₁₂-alkenylene” refers to a divalent C₄₋₁₂-alkenyl of formula —(CH₂)_(x)—(CH═CH)_(y)—(CH₂)_(z)— wherein x and z are, independently, 0, 1, 2, 3, 4, 5, 6, 7 or 8 and y is 1, 2, 3 or 4. Similarly, the term “C₄₋₈-alkenylene”, refers to a divalent C₄₋₈-alkenyl. Examples of alkenylene groups include butenyl, pentenyl, pentadienyl, hexenyl, hexadienyl, heptenyl, heptadienyl, octenyl, octadienyl, nonenyl, nonadienyl, decenyl, decadienyl, undecenyl, undecadienyl, undodecenyl, undodecadienyl, and their straight-chain and branched and stereo-isomers.

The term “C₃₋₆-cycloalkyl” refers to a saturated or partially unsaturated cyclic hydrocarbon group having 3 to 6 (inclusive) carbon atoms. This term is further exemplified by groups as cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl. The “C₃₋₆-cycloalkyl” group can optionally be mono-, di-, tri- or multiply-substituted by a halogen as defined below, a C₁₋₄-alkyl group as defined above, a —NH₂, a —NH—CO₂H, a —NH—CO—R^(a), —CO₂H, —NHR^(a) and/or —NR^(a)R^(b) wherein R^(a) and R^(b) are independently a C₁₋₄-alkyl group as defined above.

The term —(CH₂)—C₃₋₆-cycloalkyl group refers to a —CH₂— substituted C₃₋₆-cycloalkyl group wherein the cycloalkyl group is as defined above.

The term “C₆₋₁₀ aryl” refers to a monocyclic or bicyclic aromatic ring system of 6 to 10 (inclusive) carbon atoms, preferably 6 carbon atoms. This term is further exemplified by groups as phenyl and naphtyl. The C₆₋₁₀-aryl group can optionally be mono-, di-, tri- or multiply-substituted by a halogen as defined below and/or a C₁₋₄-alkyl group as defined above.

The terms “halo” or “halogen” refers to fluorine, chlorine, bromine and iodine.

The term “halogen-C₁₋₄-alkyl”, refers to a halogen substituted C₁₋₄-alkyl group wherein both halogen and alkyl groups have the meaning as above. Preferred “halogen-C₁₋₄-alkyl” groups are fluorinated “halogen-C₁₋₄-alkyl” groups such as —CF₃, —CH₂—CF₃, —CH(CF₃)₂, —CH(CH₃)(CF₃), —C₄F₉.

The term “C₁₋₁₂-alkoxy” refers to a branched or straight-chain monovalent saturated aliphatic hydrocarbon group of 1 to 12 (inclusive) carbon atoms attached to an oxygen atom. Similarly, the terms “C₁₋₈-alkoxy”, “C₁₋₅-alkoxy”, “C₁₋₄-alkoxy”, “C₁₋₂-alkoxy” refer to branched or straight-chain monovalent saturated aliphatic hydrocarbon groups of, respectively, 1 to 8 (inclusive), 1 to 5 (inclusive), 1 to 4 (inclusive), 1 to 2 carbon atoms. Examples of “alkoxy” groups are methoxy, ethoxy, propoxy, butoxy, pentoxy, hexyloxy, heptyloxy, octyloxy, and their branched isomers.

The term “C₅₋₁₀-heteroaryl” refers to a heterocyclic aryl group containing 1 to 3 heteroatoms in the ring with the remainder being carbon atoms. In the said heterocyclic aryl group, suitable heteroatoms include, without limitation, sulfur and nitrogen. Exemplary heteroaryl groups include indolyl, azaindolyl, thiophenyl, benzothiophenyl, thioazolyl, benzothiazolyl. The heteroaryl group can optionally be mono-, di-, tri- or multiply-substituted by a halogen and/or a C₁₋₄-alkyl group, as defined above. When the heteroaryl group is mono-, di-, tri- or multiply-substituted by a C₁₋₄-alkyl group, said alkyl group is preferably a methyl group.

The term “polyether ring”, refers ring containing 1, 2, or 3 ether groups, an ether group being an oxygen atom connected to two alkyl groups as defined above

The term “lactone” refers to a closed ring containing an oxygen atom adjacent to a carbonyl group (—CO—O—). It can be considered as the condensation product of an —OH group with a —CO₂H group.

The term “lactam” refers to a closed ring containing an nitrogen atom adjacent to a carbonyl group (—CO—NH— or —CO—NR— with R being for example an alkyl group as defined above).

The terms “substituted” and “substitution and the like”, refer to the replacement of one, two, three or more atoms in a given group by one, two, three or more suitable substituents, including, without limitation, a halogen, a C₆₋₁₀ aryl group, a C₁₋₄-alkyl group, a C₁₋₂-alkyl group, a C₁₋₂-alkoxy group, a —NH₂, a —NH—CO—R^(a), —CO₂H, —NHR^(a) and/or —NR^(a)R^(b) wherein R^(a) and R^(b) are independently a C₁₋₄-alkyl group, or a mixture of those substituents.

In some embodiments of the invention, the compounds of the invention can contain one or more asymmetric centers and thus occur as racemates and racemic mixtures, single enantiomers, individual diastereoisomeric mixtures. All such isomeric forms of these compounds are included in the present invention, unless expressly provided otherwise.

In some embodiments, the compounds of the invention can contain one or more double bonds and thus occur as individual or mixtures of Z and/or E isomers. All such isomeric forms of these compounds are included in the present invention, unless expressly provided otherwise.

In the embodiments where the compounds of the invention can contain multiple tautomeric forms, the present invention also includes all tautomeric forms of said compounds unless expressly provided otherwise.

In the embodiment where R^(7a) together with R⁶ form a lactone or a polyether ring,

-   -   R^(7a) is C₄₋₁₂-alkylene, preferably C₄₋₈-alkylene group, and is         linked to a —CO—O— or to a —O— functional group in R⁶, or     -   R^(7a) is C₄₋₁₂-alkenylene, preferably C₄₋₈-alkenylene group,         and is linked to a —CO—O— or to a —O— functional group in R⁶.

In the embodiment where R^(7b) together with R⁶ form a lactam, R³ is a —(CH₂)_(q)—CO—NHR^(7b) and R^(7b) is —(CH₂)_(q′)—NH— with q′ being 2, 3, 4, 5, 6, 7 or 8.

In the embodiment where R⁹ together with R⁶ form a lactam, R³ is a —(CH₂)_(q)NHR⁹ and R⁹ is a direct link between —(CH₂)_(q)NH— and a —CO— functional group in R⁶.

In the embodiment where R⁸ together with R⁶ form a lactone or a polyether ring:

-   -   R⁸ is C₄₋₁₂-alkylene, preferably C₄₋₈-alkylene group, and is         linked to a —CO—O— or to a —O— functional group in R⁶, or     -   R⁸ is C₄₋₁₂-alkenylene, preferably C₄₋₈-alkenylene group, and is         linked to a —CO—O— or to a —O— functional group in R⁶.

The terms “β ring”, “β protein” or “β clamp” herein designate the β subunit of a eubacterial DNA polymerase III, such as that of E. coli. The β subunit of DNA polymerase III of E. coli is in particular described in Kong et al. (1992)¹.

Further definitions are added in the text, when necessary.

Particular embodiments of the compounds according to the invention are described in the following more detailed specification.

According to a particular embodiment of the compounds according to the invention, the R group indicated in the above formula (I) is selected amongst a C₁₋₈-alkyl group optionally substituted by a C₆₋₁₀-aryl group, a C₂₋₈-alkenyl group optionally substituted by a C₆₋₁₀-aryl group or a C₁₋₅-alkyl-(O—CH₂—CH₂)_(t)— group with t being 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10. Preferably, R is a C₁₋₄-alkyl group optionally substituted by a C₆₋₁₀-aryl group or a C₂₋₄-alkenyl group optionally substituted by a C₆₋₁₀-aryl group. Indeed, as described in the experimental part below, the inventors have observed that a N-terminal acetylation of the peptide P5 (QLDLF, SEQ ID No: 5) leads to a 10-fold increase of the affinity of the peptide for the β interacting pocket.

When m is not null and p is >1 in the above formula (I), for example when m=1 and p=2, 3, 4, 5, 6, 7, 8, 9 or 10, the above-described compounds are pegylated. The pegylation contributes to the stability of the whole molecule and can also have a positive effect for the entry of said compound into bacterial cells.

Particular compounds according to the invention have one or several arginines and/or lysines at the N-terminal extremity of the peptide part of the compound. For example, n is 1, 2, 3, 4 or 5. Indeed, these positively charged residues are known to favor membrane crossing. This feature is however not compulsory, and other compounds according to the invention do not have such residues (n=0 in formula I).

The inventors have also observed that substitution of the second amino acid of Ac-QLDLF (SEQ ID No: 6, P6) by a beta-cyclohexyl-L-alanyl (hereafter designated as “Cha”) leads to a further 6-fold increase of the affinity of said peptide with the β hydrophobic pocket. Other substitutions at the same position (see Table 4 in the experimental part) led to the above definition of R² in formula (I). According to an advantageous embodiment, R² is a —(CH₂)—C₃₋₆-cycloalkyl group.

By substituting the leucine of the acetylated peptide Ac-Gln-Cha-Asp-Leu-Phe 7 (SEQ ID No: 7, P7) by a number of different residues (see Table 5 of the experimental part below), the inventors could determine preferable embodiments for R⁴ group, in order to optimize the binding to the interacting pocket of the β ring. Accordingly, R⁴ is preferably selected amongst C₁₋₅-alkyl groups, especially branched ones (such as valine, leucine or homoleucine lateral chains, for example), or amongst C₁₋₃—, preferably C₁₋₂-alkyl groups optionally substituted by a C₃₋₆-cycloalkyl group (such as Cha and homoCha, for example).

As shown in Table 6 below, modifications introduced on the C-terminal phenylalanine benzyl ring led to an increase of the affinity of the molecule with the interacting pocket of the β ring. The binding affinity was found to increase with the size of the ring substituent (p-methyl<p-chloro<p-bromo<3,4-dichloro). The same table shows that other cyclic molecules can be used in this position. Contrarily, replacement of the phenylalanine lateral chain by a 2-amino-tetradecanoic acid led to a significant loss in affinity, thereby indicating an upper limit for the size of the group to be used at this position. Accordingly, in the compounds of the present invention, R⁵ is preferably a —(CH₂)—C₆₋₁₀-aryl group optionally substituted by a halogen, a C₁₋₂ alkyl group and/or a C₁₋₂ alkoxy group.

As exemplified in the experimental part below, excellent affinities are obtained with linear molecules having a peptidic skeleton. Such molecules excellently mimic the binding part of the proteins which naturally interact with the β ring. When the compounds according to the invention are linear, R³ and R⁶ are as follows:

-   -   R³ is selected in the group consisting of a C₁₋₈-alkyl group,         the side chain of arginine or lysine, —(CH₂)_(q)—CO₂R^(7a),         —(CH₂)_(q)—CO—NHR^(7b), —CH₂OR⁸, —(CH₂)_(q)NHR⁹, wherein         -   q is 1, 2, 3, 4,         -   R^(7a) is a hydrogen atom, or a C₁₋₈-alkyl group,         -   R^(7b) is a hydrogen atom, or a C₁₋₈-alkyl group,         -   R⁸ is a hydrogen atom, a C₁₋₈-alkyl group,         -   R⁹ is a hydrogen atom;

and

-   -   R⁶ is —CO₂H, —CO₂R¹⁰, —CO—NH₂, —CO—NHR¹⁰, —OR¹⁰ when r is 1 or         2, —NH—CO—NHR¹⁰ when r is 1 or 2; wherein         -   R¹⁰ is a C₁₋₈-alkyl group optionally substituted by a             C₆₋₁₀-aryl group; a C₃₋₆-cycloalkyl group; a C₆₋₁₀-aryl             group optionally substituted by a halogen, a C₁₋₂-alkyl             group and/or a C₁₋₂-alkoxy group.

It is to be noted that R⁶ is directed towards the solvent. Hence, R⁶ can be replaced by virtually any kind of molecule. For example, it can advantageously be replaced by or attached to a molecule which helps the crossing of membranes and/or the internalization by the bacteria. Non-limitative examples of such molecules are cell-penetrating peptides (CPP) (Classes and prediction of cell-penetrating peptides, Lindgren M, Langel U., Methods Mol Biol. 2011, 683, p. 3-19). In case a CPP or another molecule is covalently bound to the compound via R⁶, a linker, made of one to 10, preferably 1 to 4 amino acids, can be added between the compound of the invention and said CPP. Such a linker can be, for example, a mere arginine or lysine, or a sequence of 2 to 4 amino-acids corresponding to the amino-acids immediately following the binding site of a natural ligand of the β ring, such as, for example, ASRQ (SEQ ID No: 31), which is the sequence following the binding site of the delta protein from the gamma complex. Indeed, as shown by Jeruzalmi et al (2001)^(15, 15a), this protein exhibits a bend towards the outside of the pocket. Hence, a CPP bound via a ASRQ linker to a compound according to the invention would not hinder the interaction of said compound with the β ring.

According to a particular embodiment of the linear compounds of the present invention, R³ and/or R⁶ are as follows:

-   -   R³ is selected in the group consisting of the side chain of         arginine, the side chain of lysine, —(CH₂)_(q)—CO₂R^(7a) and         —(CH₂)_(q)—CO—NHR^(7b), wherein         -   q is 1, 2, 3 or 4,         -   R^(7a) is a hydrogen atom, or a C₁₋₈-alkyl group, and         -   R^(7b) is a hydrogen atom, or a C₁₋₈-alkyl group,

and/or

-   -   R⁶ is —CO₂H or —CO—NH₂.

In an alternative embodiment, the compounds according to the present invention are cyclic, a cycle being made between the R³ and R⁶ groups. This bridge between R³ and R⁶ groups eliminates carboxylates, thereby improving the capacity of the compounds to enter bacterial cells, without impacting R5, which is necessary for anchoring the compound in the pocket, and for the subsequent conformational modification of said pocket. According to this embodiment, R³ and R⁶ are as follows:

-   -   R³ is selected in the group consisting of —(CH₂)_(q)—CO₂R^(7a),         —(CH₂)_(q)—CO—NHR^(7b), —CH₂OR⁸, —(CH₂)_(q)NHR⁹, wherein         -   q is 1, 2, 3 or 4,         -   R^(7a) is a C₄₋₈-alkylene group forming together with R⁶ a             lactone or a polyether ring, or a C₄₋₈-alkenylene, forming             together with R⁶ a lactone or a polyether ring,         -   R^(7b) is —(CH₂)_(q′)—NH— with q′ being an integer from 2 to             8 inclusive and forming together with R⁶ a lactam,         -   R⁸ is a C₄₋₈-alkylene group forming together with R⁶ a             lactone or a polyether ring, or a C₄₋₈-alkenylene, forming             together with R⁶ a lactone or a polyether ring,         -   R⁹ together with R⁶ form a lactam;     -   R⁶ is —CO—, —CO—O— or —O— and forms a lactam, a lactone, or a         polyether ring with R^(7a), R^(7b), R⁸ or R⁹.

Particular compounds according to the present invention are described in the experimental part which follows. Particular compounds having a very good to excellent affinity for the β ring are: P7 (SEQ ID No: 7), P11 (SEQ ID No: 11), P12 (SEQ ID No: 12), P13 (SEQ ID No: 13), P14 (SEQ ID No: 14), P16 (SEQ ID No: 16), P17 (SEQ ID No: 17), P23 (SEQ ID No: 23), P24 (SEQ ID No: 24), P25 (SEQ ID No: 25), P26 (SEQ ID No: 26), P27 (SEQ ID No: 27).

As described in the experimental part below and as perfectly known by skilled artisans, several techniques exist to measure the affinity of two interacting proteins. These techniques may give slightly different results. However, the relative affinity of two compounds for the β ring is not dependent from the technique used for measuring said affinities (FIG. 4B). In a preferred embodiment of the compounds according to the invention, the affinity of said compounds for the interacting pocket of the bacterial β ring is at least twice the affinity of the acetylated peptide of sequence AcQLDLF (SEQ ID No: 6, P6) with said interacting pocket.

The compounds described above can advantageously be used as antibacterial agents, since they inhibit, at least partially, the interaction between the β protein and proteins that interact therewith by binding to its hydrophobic pocket.

A pharmaceutical composition comprising, as an active agent, a compound as above-described, is also part of the present invention.

FIGURES LEGENDS

FIG. 1: Representations of the ligand binding pocket of the β ring of E. coli, from the co-crystal structure of the β ring with the C-terminal peptide of the E. coli DNA polymerase IV (R₁Q₂L₃V₄L₅G₆L₇, SEQ ID No: 32) (PDB code 1OK7). A: unbound pocket: the M₃₆₂ (Δ) residue is located close to the H₁₇₅ β residue (*) and obstructs the path between subsite 1 (black dots area) and subsite 2 (white dots area). Water molecules are represented as medium grey balls. B: bound pocket. The peptide has been removed. The movement of residue M₃₆₂ opens a cleft (dark arrow) which connects subsite 1 and subsite 2 and where the V₄ peptide residue interacts (see C). Water molecules are displaced, as compared to A, so that the peptide can fit into subsite 1. Note the opening of the platform (white star) between M₃₆₂ and R₃₆₅ where the L₃ peptide residue will be located. C: Same as B but with the peptide P1 bound into the pocket.

FIG. 2: A: Energetic contributions (Kcal/mol) of each peptide residue (R₁Q₂L₃V₄L₅G₆L₇, SEQ ID No: 32) for the interaction within the binding pocket of the β ring (PDB 1OK7). Black: electrostatic contribution, dark grey: solvent accessible surface contribution, light grey: Van der Waals contribution, white: total contribution. B: Single residue contribution (kcal/mol) to the peptide binding. Native peptide P1 of E. coli DNA polymerase IV, from the structure 1OK7, is in black. The pentapeptide P6 is in grey (PDB 3Q4J).

FIG. 3: Detailed connectivities between β residues N₃₂₀ and M₃₆₂ in subsite 2 of the binding pocket, in absence (A) or presence (B) of the peptide. Balls represent water molecules. From PDB structure 1OK7, incorporated herein by reference.

FIG. 4: Polymerase competition assay. A: the β dependant activity of PolIV DNA polymerase is challenged by increasing concentrations of various peptides B: the table displays the IC₅₀ determined for various peptides by the Pol IV based biochemical assay and the SPR assay. The histogram indicates that the same general trend is observed with both techniques despite a difference in sensitivity. Grey: biochemical assay, black: SPR assay. P15 sequence is Ac-RQLVLF, (SEQ ID No: 15), Scr: scrambled peptide: Ac-ChaFQLD, (SEQ ID No: 33).

FIG. 5: Superimposition of peptide-β complexes. A: A P6-β complex (pale colors) is superimposed on P12-β complex (dark colors) (rmsd: 0.95 Å). The first (Gln) and last (Phe) peptide residues are indicated. The Cha group of P12 (SEQ ID No: 12) peptide occupies the same position as the Leu₂ residue of P6 (SEQ ID No: 6). The chloro-modified Phe residue of P12 is tilted toward the bottom of subsite 1 as compared to the cognate residue of P6. B: P14-β complex (pale colors) is superimposed on P12-β complex (dark colors) (rmsd: 0.56 Å). The chlorine atom in meta position forms an halogen bond with T172 residue.

FIG. 6: Superposition of the peptide free (dark) and peptide bound (pale) interacting pockets of 1OK7 structure. In the absence of peptide, the M₃₆₂ side chain (dark) is located close to the H₁₇₅ residue (closed conformation), and separates subsite 1 and subsite 2. When the peptide is bound, the M₃₆₂ side chain (pale) is displaced away from the H₁₇₅ (open conformation) allowing the opening of a cleft in which the peptide can bind. Residue R₃₆₅ is also shifted upon peptide binding, triggering the opening of a small platform where the peptide L₃ residue locates.

FIG. 7: Graphical representation of the quantitative analysis of polymerase competition assays performed with several peptides. The percentage of inhibition of β dependent E. coli DNA polymerase IV activity is plotted as a function of peptide concentration (μM). P15 sequence is Ac-RQLVLF (SEQ ID No: 15). Scr: scramble peptide: Ac-ChaFQLD (SEQ ID No: 33); (related to FIG. 4).

FIG. 8: Isothermal titration calorimetry (ITC).

A. Binding isotherms for the titration of the β ring with peptide P12 (SEQ ID No: 12) and P14 (SEQ ID No: 14). N: number of sites per β monomer.

B. Enthalpy-entropy compensation for selected natural and non-natural β binding peptides. The thermodynamics parameters are determined by ITC. Each value is the mean of two independent experiments monitoring the binding of each peptide (400 μM) to the β ring (20 or 30 μM) at 25° C. Each correlation point is labeled according to the corresponding peptide, and the respective AG values are plotted below. 1 cal=4.18J; (related to Table 11).

FIG. 9: Examples of compounds according to the invention are represented in FIG. 9 (A-H). Those include compounds wherein R=acetyl, cynamoyle, octanoyle; R¹=Cl and R²=H, or R¹=R²=Cl, or R¹=R²=H, or R¹=Me and R²=H, or R¹=Br and R²=H; and R³=OH or NH₂. Specific compounds P23 (SEQ ID No: 23), P24 (SEQ ID No: 24), P25 (SEQ ID No: 25), P26 (SEQ ID No: 26), P27 (SEQ ID No: 27), P28 (SEQ ID No: 28), P29 (SEQ ID No: 29) and P30 (SEQ ID No: 30) are disclosed in FIG. 9H.

EXAMPLES Example 1 Structure-Based Design of Short Peptide Ligands Binding onto the E. coli Processivity Ring

1.1. Material and Methods

1.1.1. Protein Production, Purification and Characterization

The E. coli dnaN gene was cloned into pET15b plasmid (Invitrogen) using standard protocols. The resulting N-tagged protein was expressed in BL21 E. coli cells after IPTG induction (0.1 mM) at 28° C. The β protein fraction was first enriched on a Ni-NTA column, eluted with an histidine step (300 mM) and further purified on a MonoQ column in buffer containing 20 mM Tris HCl pH 7.5, 0.5 mM EDTA and 10% glycerol, using a gradient from 0 to 0.5 M NaCl. The quality of the protein was assessed by mass spectrometry in denaturing and native conditions

1.1.2. Peptide Synthesis

Peptides P1-P14 (SEQ ID Nos: 1 to 14) were synthesized in Fmoc chemistry by the stepwise solid-phase methodology²⁸ on a home-made semi-automatic peptide synthesizer²⁹. N—N-Fmoc protected amino acids (natural and non natural) are commercially available from Polypeptide Labs (Strasbourg, France). Resins for solid-phase peptide synthesis are commercially available from Polypeptide Labs (Strasbourg, France) and CBL Patras (Patras, Greece). Assembly of the protected peptide chains was carried out on a 100-μmol scale starting from either Fmoc-Leu-Wang resin (Peptides P1, P2, P4), Fmoc-Phe-Wang (Peptides P3, P5-P10) resin or o-chlorotrityl chloride resin (peptide P11-P14). For each coupling step, the reactants were introduced manually as a solution in dry DMF (2.0 mL). Nα-Fmoc amino acids (5.0 equivalent) with standard side-chain protecting groups were coupled 2 times by using BOP (5.0 equivalent), HOBt (5.0 equivalent) and DIEA (10.0 equiv) in dry DMF for 20 min. The washing of the resin as well as Fmoc deprotection (by using a freshly prepared solution of 20% piperidine in DMF) were performed automatically. The coupling and deprotection steps were monitored by the Kaiser test³⁰. At the end of the elongation of the peptidic chain, the resin was washed with CH₂Cl₂ and dried with Et₂O. A mixture of TFA/H₂O/TIPS/DTT (8.8/0.5/0.2/0.5; 10.0 mL) was then added to the resin. The mixture was gently shaken for 2.5 h and the resulting solution was flushed through a frit in cold Et₂O. The precipitate was recovered by centrifugation, dissolved in a mixture of AcOH and H₂O and freeze-dried. The crude peptides were finally purified by HPLC (linear gradient, 5-65% B, 30 min) and freeze-dried. All peptides were identified by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), and their homogeneity was assessed by C₁₈ RP-HPLC (purity of all peptides determined to be >90%).

Analytical data are reported in Table 8.

1.1.3. Molecular Dynamics

In the present work, the inventors used a protocol³¹ based on the MM/PBSA method^(32, 33), where conformations extracted from molecular dynamics simulations are processed using a simplified description for the solvent to yield an estimate of binding free energy. Individual contributions of each amino acid to the complex formation are estimated and important energetic amino acid “hot spots” are identified.

Structures

The initial structure for the apo protein was chain A from the PDB file 1OK7¹⁷, while for the protein and native peptide it was chains B and C from the same PDB (1OK7). All crystallographic water molecules were retained.

MD Simulations

The CHARMM program³⁴, version 32, with the CHARMM 22 all atom protein-nucleic acid force field³⁵ was used for the molecular dynamics simulations. Hydrogen atoms were added using the HBUILD facility in the CHARMM program. A sphere of 37 Å containing 6840 water molecules (TIP3) was used to solvate the system. Stochastic boundary conditions were imposed and the calculation was limited to residues 7 Å around the peptide. The SHAKE algorithm was used to constrain hydrogen-heavy atom bond distances, and the simulations were done using Langevin algorithm. A 1-fs time step was used for the molecular dynamics simulation and the simulation time. A 12 Å cutoff was used; the van der Waals non bonded terms were treated with a SWITCH potential function whereas the electrostatic terms was evaluated with the SHIFT function.

Free Energy Decomposition of Interactions Between the E. coli β Clamp and the Different Peptides.

To obtain a semi-quantitative estimate of the contributions of all amino acids to the binding free energy for the formation of the β clamp-peptide complex, a molecular free energy decomposition scheme based on the Molecular Mechanics/Poisson-Boltzmann Surface Area (MM/PBSA) analysis was performed, following the approach presented by Lafont et al.³¹. From this analysis, an estimation of the free energy of binding for molecular complexes can be obtained. Briefly, in the MM/PBSA approach, the free energy is estimated using a standard thermodynamic cycle of the form

where the binding free energy is calculated according to the equation: ΔG _(assoc) ^(solution) =ΔE _(MM) ^(gas) −TΔS+ _(MM) +ΔG _(solvation)

where Δ_(MM) ^(gas) is the difference in the gas phase energy; ΔS_(MM) is the change in entropy upon complex formation and ΔG_(solvation) is the change in solvation free energy. The gas phase energy differences contain terms for the intermolecular electrostatic and van der Waals energies, as well as the equivalent internal energy terms. These terms are based on the CHARMM force field in the present approach. The solvation free energy is divided into two contributions: an electrostatic and a nonpolar contribution. This latter term is approximated by an empirical relationship based on solvent accessible surface area and the electrostatic contribution is calculated here using the Poisson-Boltzmann equation.

Several approximations are introduced in the MM/PBSA method. The first was the neglect of conformational change upon complex formation, which is dictated by the absence of experimental structures for the unbound protein and peptides. To account for the unbound species in the calculations, their respective structures were obtained from the complex generated during the molecular dynamics simulations. With this approximation, there are no changes to the internal energy terms. The second approximation is the neglect of changes in configuration entropy due to binding. Although these simplifications preclude calculations of absolute values of the binding free energies, they have been shown in previous work to be satisfactory in the context of identifying interaction energy “hot spots” in protein-protein and protein-ligand complexes. Similar simplifications have been employed in other studies³⁶ ³¹ ³⁷. Via this approach, the total binding free energy can be decomposed into individual energetic contributions per residue. Decomposition of the binding free energy to individual amino acid contributions leads to the identification of amino acids that play a dominant role in binding and can contribute to reliable predictions of the role of particular amino acids in stabilizing complexes.

1.1.4. Structure-Based Design of Peptides

From the initial structural and energetic analysis of the RQLVLGL (SEQ ID No: 1, P1 in Table 1) peptide binding to the β clamp, modification of the sequence appeared potentially interesting in three positions (cf. FIG. 1): Q2; L3 and the hydrophobic L5-G6-L7 segment. In order to identify interesting modifications, the programs MCSS³⁸ and SEED³⁹ were used to dock small librairies of hydrophobic and polar small ligands (fragments) onto the surface of the β-clamp encompassing the peptide binding site. The protocol incorporated improved scoring functions with solvation corrections.^(40, 41). From this initial step, it appeared difficult to find replacements for the Q2 side-chain of the peptide that would correctly maintain the intricate hydrogen-bond network at this position (see FIG. 3) and therefore no modification of Q2 were attempted. For the other positions, improving interactions with optimized hydrophobic contacts appeared promising. Based on these initial data, a selection of peptides with modified side chains were constructed, docked into the structure and their interactions with the β clamp evaluated using the MM-PBSA protocol described above. The choice of side-chain replacements was based on the docking data, focusing on commercially available protected amino-acids. A force field adapted from CHARMM 22³⁵ was used for non-natural amino acids. The most promising candidates were selected for synthesis.

1.1.5. β/Peptide Interaction in Solution: In Vitro Competition Assays.

5′ end radiolabelling, purification and annealing of synthetic primers were performed as previously described (Wagner et al., 1999). The 30/90mer synthetic construct was obtained by annealing the 30 mer primer (5′GTAAAACGACGGCCAGTGCCAAGCTTAGTC3′, SEQ ID No: 34) with the 90 mer template (5 ‘CCATGATTACGAATTCAGTCATCACCGGCGCCACAGACTAAGCTTGGCACTG GCCGTCGTTTTACAACGTCGTGACTGGGAAAACCCTGG3’, SEQ ID No:35) to form a double stranded structure with 5′ and 3′ ssDNA overhangs of 25 and 35 nucleotides, respectively. All replication experiments (10 μl final volume) were carried out in buffer E (40 mM HEPES pH 7.5, 80 mM potassium glutamate, 160 μg/ml BSA, 16% glycerol, 0.016% NP40, 8 mM DTT). The 30/90 mer duplex (1 nM final concentration) was first incubated with single strand binding protein (SSB; Sigma; 90 nM final concentration) in the presence of ATP (200 μM) and MgCl₂ (7.5 mM) at 37° C. for 10 min. γ complex (1 nM final concentration) (obtained as described by Dallmann et al, 1995) and β clamp (5 nM as dimer final concentration) were added at that stage and incubation was carried out at 37° C. for 10 min. Then, 7 μl of the mixture was added to 1 μl of either DMSO or 1 μl of peptide solution (as specified), incubated 5 min. at room temperature and further 2 hours at 4° C. 1 μl of PolIV was then added (1.5 nM final concentration), incubated 5 min. at room temperature and finally, the whole reaction was mixed with 1 μl of a dNTPs solution (200 μM each dNTP final concentration) and let react for 1 min. at room temperature. Reactions were quenched by the addition of 20 μl of 95% formamide/dyes solution containing 7.5 mM EDTA, heat-denatured and analysed by chromatography on 12% denaturing polyacrylamide gels. Radiolabelled products were visualised and quantified using a Personal Molecular Imager Fx and the Quantity One software (Bio-Rad).

1.1.6. SPR Assays.

SPR experiments were performed on a Biacore® 3000. The association constant (K_(A)) of β with the natural Cter heptamer (P1, Table 7) of the DNA polymerase IV of E. coli were determined as follow: the β protein (0.125 μM to 2 μM) was injected on the immobilized P1 peptide at a flow rate of 500 μL·min⁻¹. After subtraction of the background response, the data were fit to the 1:1 Langmuir model using BIAevaluation (Biacore™). The inhibition of P1-β interaction by peptides P2 to P14 (Table 7) was used to measure their affinity for β and was assessed according to the following procedure: complexes of β ring (0.25 μM) with various concentrations of challenging peptides (1.5 nM to 100 μM) were formed and injected on a chip loaded with the P1 peptide. IC₅₀ values for each challenging peptide were determined by plotting the concentration of peptide against the percentage of binding inhibition. The IC₅₀ value of each peptide was used to calculate Ki (Ki=(1+K_(A)[β])/IC₅₀) which measures the affinity of the challenging peptide for β in the competition assay, and AG was derived from Ki (ΔG=RT ln Ki).

1.1.7. Isothermal Titration Calorimetry.

ITC was performed by using a ITC200 microcalorimeter from MicroCal. Peptides (400 μM) were titrated in sequential injections (2 μl each) into a β ring solution (300 μl, 20 or 30 μM) at 25° C. Data were corrected from control experiments where peptides were injected in buffer solution (Hepes 10 mM pH 7.4, NaCl 0.15M, EDTA 3 mM, P20 0.005%). Data analysis was performed with Origin 7.0 software.

1.1.8. Crystallogenesis, Data Collection and Processing.

Crystallization experiments were essentially conducted as described previously¹⁷. Crystals of P12-β complexes were grown in capillaries in presence of 0.2% of agarose⁴². Crystallisation buffer contained 100 mM CaCl₂, 100 mM Mes pH 6.0 and 30% PEG 400. Cryoprotection was performed by soaking crystals in the same buffer supplemented with 20% glycerol. Cryoprotected crystals were frozen in liquid ethane and X-ray diffraction data were collected at 100 K at beamline X06SA at the Swiss Light Source (Villigen PSI, Switzerland) and beamlines ID29 and ID14-4 at ESRF (Grenoble, France). Diffraction images were processed with XDS, XSCALE and XDSCONV⁴³. The structures were solved by molecular replacement with MOLREP⁴⁴, using the known beta structure as a search model PDB ID 1OK7¹². Alternate rounds of rebuilding and refinement, including noncrystallographic symmetry restraints, were carried out with PHENIX⁴⁵, COOT⁴⁶ and CNS⁴⁷. Model statistics were obtained with Molprobity⁴⁸. Molecular visualizations and structures illustrations were performed using PyMOL⁴⁹. Data processing and refinement statistics are summarized in Table 11.

1.2. Results

1.2.1. Structure and Energetics of the Binding Pocket

The inventors have previously solved the structure of a complex formed between the E. coli β clamp and the 16 residues long C-terminal peptide of the E. coli DNA polymerase IV (PDB code: 1OK7)¹⁷. A first part of the present work aimed at unraveling the molecular basis of the peptide-pocket interactions. Molecular modeling approaches were used to determine the contribution of each residue of the last seven amino-acids of the C-terminal part of Pol IV (R₁Q₂L₃V₄L₅G₆L₇, SEQ ID No: 32) to the overall interaction (FIG. 2A), using free energy decomposition (see Material and Methods). For each amino acid, the van der Waals, electrostatic and hydrophobic solvation contribution to binding have been calculated. Stabilizing interactions between the β ring and the peptide are essentially Van der Waals contacts (see FIG. 2A). Electrostatics contributions are poor, due to compensation between the protein-peptide interaction and the peptide desolvation cost. Hydrophobic solvation contributions are favorable but of lesser magnitude. The net contributions of residues Q2, L3, L5 and L₇ is predominant to the overall interaction (FIG. 2B). G₆ has no contribution while V₄ which is oriented toward the solvent poorly contributes to the interaction.

Due to the good resolution, the inventors could also analyze the position of water molecules in the free and bound pockets of the 1OK7 structure. In the absence of peptide, four water molecules are located in subsite 1. Upon peptide binding, one is eliminated and one is repositioned close to the T₁₄₂ and Y₁₅₄ residues, allowing the L₅-G₆-L₇ tripeptide to bind into the hydrophobic subsite 1 (FIG. 1AB). The two water molecules located on the platform in the apo monomer are dislodged upon peptide binding, thus making room for the peptide L₃ residue to bind (FIG. 1BC). Finally, two water molecules are deeply inserted into the empty subsite 2. One of these two molecules interconnects the N_(α)H of N₃₂₀ and the C_(α)=O of M₃₆₂ (FIG. 3) and is not exchanged with the solvent upon peptide binding, underlining its structural function. The second water molecule is replaced by the Cδ=0 of peptide Q2 residue, while its δ-amino group establishes bounds with the Cα=O of M₃₆₂ and the Cα=O of peptide residue L₂ (FIG. 3B).

This initial analysis led the inventors to design a minimal peptide binding sequence that was used as a starting point for ligand optimization. Because of the complex network of hydrogen bonds formed by the highly conserved Q residue in subsite 2, one cannot substitute this side chain without dramatically altering the interaction of the whole peptide. Alternatively, several other positions in the peptide sequence may accept modifications that could increase its affinity for the β clamp. Following the structural and energetic analysis of the binding pocket (see Material and Methods), several peptides were synthesized (Table 7 and Table 11) and their binding efficiencies were analyzed by surface plasmon resonance (SPR). The dissociation constant of the P1 natural heptapeptide was measured to be 2.85 (±0.94) 10⁻⁷ M. As compared to the whole polymerase, this peptide binds 30 fold less efficiently to the ring (table 1, compare P1 and PolIV), pinpointing the contribution of alternate regions of the enzyme to the interaction^(16, 14, 19) Removing the G residue of the terminal tripeptide (-LGL) results in a two to three fold decrease in interaction (table 1, compare P1 and P2), while replacing the terminal tripeptide with the consensus LF dipeptide does not affect the affinity (table 1, compare P1 and P3). However, a FL dipeptide totally disrupts peptide binding (table 1, P4). Substituting F for other aromatic residues (W, Y) at the C-terminal position does not contribute to any increased interaction (data not shown). In order to design the shortest peptide, the first (R₁) residue was also removed, which does not seem to contribute significantly to the binding (table 1, P5, FIGS. 1 and 2)¹⁷, and the V₄ was replaced by a D residue, as observed in the consensus sequence, in order to increase the solubility of the resulting pentapeptide P5 (QLDLF). Although its affinity for the β ring is low, it was increased by 10 fold upon acetylation (table 1, compare P5 and P6), thus providing a good compromise between interaction efficiency and ligand size.

TABLE 1 Influence of the C-terminal tripeptide sequence  and effect of N-terminal acetylation on the   interaction of peptide with the E. coli β    clamp, as measured by SPR experiments. se- IC₅₀  Ki  ΔG (Kcal/ Seq Id  # quence (μM) (10⁶ M⁻¹) mol) No: PolIV 0.29 4.7 −9.09 P1 RQLVLGL 8.85 0.15 −7.06 1 P2 RQLVLL 21.53 0.0063 −6.54 2 P3 RQLVLF 8.62 0.15 −7.04 3 P4 RQLVFL 256 Ø Ø 4 P5 QLDLF 12.44 0.11 −6.87 5 P6 AcQLDLF 1.12 1.2 −8.22 6 Ø: not determined. Ki = (1 + K_(A)[β])/IC₅₀. ΔG = −RT ln Ki. PolIV: E coli DNA polymerase IV.

1.2.2. Crystal Structure of the P6-β Ring Complex.

The P6 peptide (AcQLDLF, SEQ ID No: 6) co-crystallized with the β ring in conditions similar to those previously described¹⁷ but the cell parameters lead to a V_(M) value of 7.8, which corresponds to the presence of 3 dimers per asymmetric unit (Table 2). This structure was solved by molecular replacement at 2.3 Å resolution, using our previously determined structure (PDB 1OK7). The superposition of main chain atoms of each ring to the model led to rmsd values ranging from 0.70 Å to 1.06 Å, underlining the close structural similarity of each dimer. Each monomer of the three rings binds a peptide, and all ligands adopt a similar conformation in all six hydrophobic pockets, as indicated by a rmsd value ranging between 0.25 Å to 0.51 Å.

TABLE 2 Statistics on Data Collection and Refinement (related to FIG. 5). Beta-P14 Beta-P6 Beta-P12 Structure (PDB 3Q4L) (PDB 3Q4J) (PDB 3Q4K) Data Collection Space Group P1 P1 P1 Unit cell a (Å) 34.84 35.09 36.25 b (Å) 79.57 132.87 80 c (Å) 81.64 137.27 82.18 α (°) 65.28 62.73 66.15 β (°) 75.26 88.51 74.94 γ (°) 82.22 89.77 82.03 Beamline ID29/ESRF X06SA/SLS ID14-4/ESRF Wavelength (Å) 0.97623 0.915694 0.9794 Resolution limits (Å) 39.2-1.95 29.5-2.3 19.9-2.6 high resolution shell  2.0-1.95 2.35-2.3 2.65-2.6 Reflections: measured 221062 296785 42708 unique 54138 96508 22982 Completeness (%) 96.3 (93.8)* 98.6 (97.9)* 91.9 (60.5)* R_(merge) 0.06 (0.72)* 0.05 (0.36)* 0.067 (0.11)*  I/σ  13 (1.9)* 22.6 (3.6)*  8.7 (4.0)* Refinement Reflections R_(cryst)/R_(free) 54134/2750 96493/7742 22979/1160 R_(cryst) (%) 20.1 21.6 25.9 R_(free) (%)^(†) 23.2 25.0 30.6 Protein atoms 5579 17085 5471 Ligand atoms 106 196 103 Water molecules 299 357 129 Average B factor (Å²) Protein 33.3 52.1 30.4 Ligand 39.4 66.5 27.1 Water 40.1 44.5 27.6 R.m.s.d. bond length 0.01 0.009 0.008 (Å) R.m.s.d. angles length 1.13 1.15 1.11 (°) *Values in parentheses correspond to high resolution shell in data collections. ^(†)5% of the reflections were set aside for an Rfree test before initiating any refinement

The atomic coordinates of the peptide and the peptide binding site of the β clamp (residues ≦5 Å from the ligand) are disclosed in the following Table 3. The other residues have the same positions as in the previously determined structure (PDB 1OK7) also described in U.S. Pat. No. 7,635,583.

TABLE 3 Atomic coordinates of P6 residues and of the residues involved in the binding of P6 to the β clamp, in the crystal of P6 peptide co-crystallized with the β ring. ATOM 1 NE ARG B 152 10.195 −25.903 12.978 1.00 60.02 N ATOM 2 CZ ARG B 152 9.832 −25.010 13.891 1.00 70.75 C ATOM 3 NH1 ARG B 152 10.045 −23.710 13.686 1.00 51.13 N1+ ATOM 4 NH2 ARG B 152 9.228 −25.402 15.004 1.00 66.21 N TER 5 ARG B 152 ATOM 6 CG LEU B 155 6.034 −25.353 10.551 1.00 37.71 C ATOM 7 CD1 LEU B 155 6.887 −24.861 11.676 1.00 37.67 C ATOM 8 CD2 LEU B 155 4.805 −25.976 11.115 1.00 31.46 C TER 9 LEU B 155 ATOM 10 CB THR B 172 1.710 −23.748 14.242 1.00 31.63 C ATOM 11 CG2 THR B 172 2.028 −25.084 13.527 1.00 27.98 C ATOM 12 OG1 THR B 172 2.665 −23.450 15.241 1.00 32.30 O ATOM 13 C ASP B 173 6.157 −21.665 14.133 1.00 35.44 C ATOM 14 N GLY B 174 5.672 −22.551 14.996 1.00 34.81 N ATOM 15 CA GLY B 174 6.511 −23.182 16.011 1.00 35.62 C ATOM 16 C GLY B 174 6.492 −22.492 17.359 1.00 39.39 C ATOM 17 O GLY B 174 6.970 −23.064 18.344 1.00 39.92 O ATOM 18 N HIS B 175 5.986 −21.242 17.411 1.00 34.18 N ATOM 19 CA HIS B 175 5.900 −20.479 18.650 1.00 33.72 C ATOM 20 C HIS B 175 4.476 −20.329 19.088 1.00 35.43 C ATOM 21 O HIS B 175 4.175 −20.368 20.282 1.00 34.79 O ATOM 22 CB HIS B 175 6.562 −19.119 18.513 1.00 36.12 C ATOM 23 CG HIS B 175 7.984 −19.194 18.096 1.00 41.64 C ATOM 24 CD2 HIS B 175 9.032 −19.835 18.668 1.00 44.85 C ATOM 25 ND1 HIS B 175 8.394 −18.617 16.936 1.00 45.18 N ATOM 26 CE1 HIS B 175 9.678 −18.899 16.829 1.00 45.14 C ATOM 27 NE2 HIS B 175 10.115 −19.589 17.878 1.00 45.20 N ATOM 28 N ARG B 176 3.593 −20.133 18.121 1.00 31.08 N ATOM 29 CA ARG B 176 2.181 −19.986 18.371 1.00 29.68 C ATOM 30 C ARG B 176 1.413 −20.789 17.353 1.00 32.45 C ATOM 31 O ARG B 176 1.918 −21.053 16.262 1.00 32.86 O ATOM 32 N LEU B 177 0.240 −21.239 17.733 1.00 27.89 N ATOM 33 CA LEU B 177 −0.619 −22.029 16.875 1.00 27.83 C ATOM 34 CB LEU B 177 −0.579 −23.523 17.307 1.00 27.38 C ATOM 35 CG LEU B 177 −1.466 −24.512 16.510 1.00 31.18 C ATOM 36 CD1 LEU B 177 −0.745 −25.845 16.280 1.00 30.87 C TER 37 LEU B 177 ATOM 38 CB PRO B 242 3.262 −29.933 14.003 1.00 42.35 C ATOM 39 CG PRO B 242 3.185 −28.774 13.112 1.00 45.82 C ATOM 40 CD PRO B 242 3.325 −29.320 11.745 1.00 40.86 C TER 41 PRO B 242 ATOM 42 O VAL B 247 −0.058 −27.602 22.929 1.00 52.04 O ATOM 43 CB VAL B 247 0.470 −28.149 19.728 1.00 50.73 C ATOM 44 CG1 VAL B 247 0.574 −26.655 20.015 1.00 50.50 C ATOM 45 CG2 VAL B 247 1.641 −28.626 18.875 1.00 51.23 C TER 46 VAL B 247 ATOM 47 O GLY B 318 5.474 −15.393 28.086 1.00 28.75 O ATOM 48 N PHE B 319 5.344 −13.225 27.583 1.00 24.23 N ATOM 49 CA PHE B 319 4.851 −13.489 26.241 1.00 24.99 C ATOM 50 C PHE B 319 5.356 −12.468 25.290 1.00 30.49 C ATOM 51 O PHE B 319 5.591 −11.324 25.669 1.00 32.74 O ATOM 52 CB PHE B 319 3.310 −13.478 26.174 1.00 27.25 C ATOM 53 CG PHE B 319 2.640 −14.732 26.670 1.00 28.63 C ATOM 54 CD1 PHE B 319 2.741 −15.919 25.957 1.00 30.47 C ATOM 55 CE1 PHE B 319 2.128 −17.094 26.424 1.00 31.60 C ATOM 56 N ASN B 320 5.468 −12.865 24.025 1.00 26.27 N ATOM 57 CA ASN B 320 5.720 −11.953 22.949 1.00 26.75 C ATOM 58 C ASN B 320 4.315 −11.306 22.760 1.00 28.81 C ATOM 59 O ASN B 320 3.351 −11.990 22.409 1.00 25.74 O ATOM 60 CB ASN B 320 6.143 −12.740 21.690 1.00 31.09 C ATOM 61 CG ASN B 320 6.252 −11.902 20.458 1.00 38.50 C ATOM 62 ND2 ASN B 320 7.226 −12.202 19.631 1.00 36.32 N TER 63 ASN B 320 ATOM 64 CB TYR B 323 2.398 −14.188 20.062 1.00 29.74 C ATOM 65 CG TYR B 323 3.671 −14.541 19.312 1.00 34.89 C ATOM 66 CD2 TYR B 323 4.613 −15.405 19.867 1.00 36.98 C ATOM 67 CE2 TYR B 323 5.769 −15.758 19.176 1.00 38.52 C ATOM 68 CZ TYR B 323 5.978 −15.280 17.899 1.00 45.00 C ATOM 69 OH TYR B 323 7.102 −15.660 17.220 1.00 51.04 O TER 70 TYR B 323 ATOM 71 O SER B 343 6.499 −19.652 31.418 1.00 42.19 O ATOM 72 CA VAL B 344 7.142 −22.358 31.029 1.00 30.95 C ATOM 73 C VAL B 344 6.382 −23.225 30.039 1.00 37.16 C ATOM 74 O VAL B 344 6.960 −23.833 29.135 1.00 38.77 O ATOM 75 CB VAL B 344 8.406 −23.037 31.630 1.00 34.02 C ATOM 76 CG1 VAL B 344 9.318 −22.002 32.284 1.00 33.14 C TER 77 VAL B 344 ATOM 78 CB SER B 346 1.690 −23.500 25.230 1.00 34.25 C ATOM 79 OG SER B 346 0.915 −24.661 25.493 1.00 39.01 O TER 80 SER B 346 ATOM 81 C VAL B 360 −0.613 −20.918 21.452 1.00 27.37 C ATOM 82 O VAL B 360 −0.111 −20.800 20.340 1.00 24.84 O ATOM 83 CB VAL B 360 −1.624 −23.300 21.499 1.00 27.65 C ATOM 84 CG1 VAL B 360 −0.575 −23.807 22.494 1.00 27.63 C ATOM 85 C VAL B 361 1.982 −19.886 23.474 1.00 27.90 C ATOM 86 CG1 VAL B 361 1.873 −16.988 22.556 1.00 22.99 C ATOM 87 N MET B 362 3.180 −20.112 23.023 1.00 28.61 N ATOM 88 CA MET B 362 4.274 −20.561 23.871 1.00 28.56 C ATOM 89 C MET B 362 4.839 −19.321 24.530 1.00 31.58 C ATOM 90 O MET B 362 5.039 −18.292 23.870 1.00 29.49 O ATOM 91 CB MET B 362 5.340 −21.302 23.049 1.00 31.41 C ATOM 92 CG MET B 362 6.222 −22.193 23.888 1.00 35.60 C ATOM 93 SD MET B 362 5.377 −23.603 24.664 1.00 38.96 S ATOM 94 CE MET B 362 6.619 −24.060 25.847 1.00 34.70 C ATOM 95 N PRO B 363 5.071 −19.362 25.842 1.00 29.24 N ATOM 96 CA PRO B 363 5.609 −18.178 26.510 1.00 28.99 C ATOM 97 C PRO B 363 7.074 −17.892 26.226 1.00 33.83 C ATOM 98 O PRO B 363 7.743 −18.614 25.456 1.00 33.34 O ATOM 99 CB PRO B 363 5.341 −18.479 27.991 1.00 30.94 C ATOM 100 CG PRO B 363 5.412 −19.947 28.091 1.00 35.37 C ATOM 101 CD PRO B 363 4.870 −20.473 26.798 1.00 31.25 C ATOM 102 N MET B 364 7.545 −16.777 26.784 1.00 29.71 N ATOM 103 CA MET B 364 8.945 −16.382 26.731 1.00 30.02 C ATOM 104 C MET B 364 9.502 −16.624 28.124 1.00 40.88 C ATOM 105 O MET B 364 8.772 −16.499 29.120 1.00 40.62 O ATOM 106 CB MET B 364 9.118 −14.915 26.403 1.00 30.48 C ATOM 107 CG MET B 364 8.757 −14.585 25.034 1.00 32.54 C ATOM 108 SD MET B 364 8.724 −12.808 24.682 1.00 35.07 s ATOM 109 CE MET B 364 10.528 −12.292 24.937 1.00 31.33 C ATOM 110 N ARG B 365 10.767 −17.037 28.190 1.00 42.29 N ATOM 111 CA ARG B 365 11.463 −17.315 29.447 1.00 44.24 C ATOM 112 C ARG B 365 11.620 −15.996 30.209 1.00 47.68 C ATOM 113 O ARG B 365 12.039 −14.991 29.621 1.00 46.53 O ATOM 114 CB ARG B 365 12.812 −18.035 29.173 1.00 49.79 C ATOM 115 CG ARG B 365 13.354 −18.871 30.335 1.00 61.58 C ATOM 116 CD ARG B 365 12.589 −20.152 30.620 1.00 75.54 C ATOM 117 NE ARG B 365 13.073 −21.279 29.817 1.00 90.27 N ATOM 118 CZ ARG B 365 12.957 −22.563 30.161 1.00 100.27 C ATOM 119 NH1 ARG B 365 13.409 −23.516 29.355 1.00 79.16 N1+ ATOM 120 NH2 ARG B 365 12.398 −22.901 31.318 1.00 86.12 N TER 121 ARG B 365 HETATM 122 O HOH B 384 8.833 −14.385 20.130 1.00 33.25 O HETATM 123 O HOH B 407 10.652 −12.727 21.066 1.00 31.86 O HETATM 124 O HOH B 465 12.648 −14.060 22.219 1.00 36.65 O HETATM 125 O HOH B 466 13.941 −12.371 23.870 1.00 26.96 O HETATM 126 C ACE H 69 12.190 −16.728 25.287 1.00 47.99 C HETATM 127 O ACE H 69 11.809 −17.683 25.955 1.00 46.88 O HETATM 128 CH3 ACE H 69 13.141 −15.743 25.924 1.00 48.05 C ATOM 129 N GLN H 70 11.778 −16.484 24.012 1.00 43.64 N ATOM 130 CA GLN H 70 10.826 −17.283 23.246 1.00 42.45 C ATOM 131 C GLN H 70 11.026 −18.818 23.243 1.00 48.79 C ATOM 132 O GLN H 70 11.987 −19.340 22.644 1.00 49.83 O ATOM 133 CB GLN H 70 10.668 −16.743 21.816 1.00 43.26 C ATOM 134 CG GLN H 70 9.503 −17.399 21.019 1.00 48.43 C ATOM 135 CD GLN H 70 8.133 −17.259 21.688 1.00 51.59 C ATOM 136 NE2 GLN H 70 7.769 −16.030 22.096 1.00 32.22 N ATOM 137 OE1 GLN H 70 7.418 −18.250 21.901 1.00 38.15 O ATOM 138 N LEU H 71 10.077 −19.531 23.889 1.00 44.09 N ATOM 139 CA LEU H 71 10.074 −20.993 23.930 1.00 42.82 C ATOM 140 C LEU H 71 9.337 −21.541 22.690 1.00 47.21 C ATOM 141 O LEU H 71 8.603 −20.812 22.008 1.00 46.65 O ATOM 142 CB LEU H 71 9.507 −21.552 25.254 1.00 42.99 C ATOM 143 CG LEU H 71 10.264 −21.176 26.550 1.00 48.52 C ATOM 144 CD1 LEU H 71 9.369 −21.316 27.773 1.00 48.02 C ATOM 145 CD2 LEU H 71 11.512 −22.045 26.736 1.00 53.09 C ATOM 146 N ASP H 72 9.557 −22.813 22.379 1.00 45.35 N ATOM 147 CA ASP H 72 8.966 −23.458 21.213 1.00 45.33 C ATOM 148 C ASP H 72 7.805 −24.369 21.558 1.00 46.72 C ATOM 149 O ASP H 72 7.847 −25.031 22.589 1.00 44.98 O ATOM 150 CB ASP H 72 10.057 −24.179 20.397 1.00 47.71 C ATOM 151 CG ASP H 72 10.805 −23.219 19.472 1.00 73.93 C ATOM 152 OD1 ASP H 72 11.558 −22.358 19.986 1.00 75.90 O ATOM 153 OD2 ASP H 72 10.576 −23.278 18.231 1.00 87.47 O1− ATOM 154 N LEU H 73 6.766 −24.394 20.694 1.00 44.17 N ATOM 155 CA LEU H 73 5.598 −25.260 20.839 1.00 44.82 C ATOM 156 C LEU H 73 5.949 −26.725 20.585 1.00 49.95 C ATOM 157 O LEU H 73 5.343 −27.628 21.189 1.00 49.17 O ATOM 158 CB LEU H 73 4.487 −24.829 19.868 1.00 44.08 C ATOM 159 CG LEU H 73 3.484 −23.817 20.345 1.00 45.47 C ATOM 160 CD1 LEU H 73 2.433 −23.649 19.295 1.00 45.34 C ATOM 161 CD2 LEU H 73 2.807 −24.251 21.651 1.00 38.54 C ATOM 162 N PHE H 74 6.897 −26.941 19.644 1.00 47.70 N ATOM 163 CA PHE H 74 7.378 −28.265 19.211 1.00 48.65 C ATOM 164 C PHE H 74 8.752 −28.089 18.510 1.00 75.49 C ATOM 165 O PHE H 74 9.100 −26.937 18.126 1.00 77.08 O ATOM 166 CB PHE H 74 6.340 −28.932 18.271 1.00 49.99 C ATOM 167 CG PHE H 74 5.819 −28.030 17.171 1.00 50.87 C ATOM 168 CD1 PHE H 74 6.502 −27.909 15.963 1.00 52.73 C ATOM 169 CD2 PHE H 74 4.661 −27.281 17.352 1.00 52.30 C ATOM 170 CE1 PHE H 74 6.047 −27.044 14.972 1.00 52.86 C ATOM 171 CE2 PHE H 74 4.230 −26.379 16.375 1.00 53.68 C ATOM 172 CZ PHE H 74 4.918 −26.281 15.186 1.00 51.51 C ATOM 173 OXT PHE H 74 9.469 −29.102 18.345 1.00 100.17 O1− TER 174 PHE H 74 HETATM 175 O HOH H 86 5.592 −15.725 23.553 1.00 33.16 O END

A free energy decomposition analysis (see Material and Methods for details) of this complex was performed (FIG. 2B) and the most important interactions are similar to the initial complex 1OK7, as expected. The canonical sequence LF advantageously replaces the LGL sequence in C-ter of the peptide (FIG. 2B). The P6 peptide acetyl group also forms two hydrogen bonds with the Nα of residues R₃₆₅ and L₃₆₆ of the β monomer which probably account for the 10 fold increase in stability of the P6 peptide as compared to P5 (Table 1). Despite its reduced size, the P6 peptide therefore has an increased affinity for the β-clamp with respect to the original peptide P1.

1.2.3. Design of Non-Natural Peptides Ligands with Increased Binding Affinity.

P6 was further used as a lead to introduce modifications aimed at increasing the affinity of the ligand for the β clamp. Because the natural ligand binds to the pocket essentially through hydrophobic interactions, the aim was to extend the network of such interactions. A first set of modifications concerned position 2, where the leucine residue was replaced by a cyclohexyl-L-alanyl group (Cha) (P7, table 4 and table 7). An initial modeling analysis, using programs MCSS and SEED, indicated that this modification provides a Van der Waals energy contribution two-fold higher than that with the natural L₃ residue, and is the most efficient group tested (Table 4). It also results in a 6 fold increase in the interaction, as measured by SPR (Table 4 and Table 7, compare P6 and P7). Attempts to increase the side chain length resulted in a drastic reduction of the affinity (Table 4 and Table 7, P8) while other modifications like homoleucyl (hLeu) or neopentylglycyl (NptGly, also called tertiomethylbutylglycine, tBMG) did not yield any gain in affinity (Table 4 and Table 7, P9 and P10), probably indicating that the area available for an efficient interaction is limited.

TABLE 4 Effect of site-specific modifications of the second residue on the interaction  of peptide with the E. coli β clamp. # sequence IC₅₀ (μM) SEQ ID No: P6 AcQLDLF 1.12 6 P7 Ac Q ChaDLF 0.17 7 P8 Ac Q hCha DLF 82.8 8 P9 Ac Q hLeu DLF 0.74 9 P10 Ac Q NptGly DLF 0.99 10 Cha: cyclohexylalanine, hCha: homocyclohexylalanine, hLeu: homoleucine, NptGly: neopentylglycyl.

A second set of similar modifications was introduced at position L₄, but no increase in affinity was observed as compared to P7 (Table 5).

TABLE 5 Effect of site-specific modifications   of the L₄, residue on the interaction  of peptide with the E. coli β clamp. # sequence IC₅₀ (μM) SEQ ID No: P6 AcQLDLF 1.12  6 P7 Ac Q Cha DLF 0.17  7 P16 Ac Q Cha D hLeu F 0.23 16 P17 Ac Q Cha D Cha F 0.31 17 P18 Ac Q Cha D hCha F 0.72 18 P19 Ac Q ChaD NptGly F 5.63 19

Finally, several modifications were introduced on the terminal phenylalanine benzyl ring (Table 6). The binding affinity was found to increase significantly with the size of the ring substituent (p-methyl<p-chloro<p-bromo<3,4-dichloro)(see P11, P12, P13, and P14, respectively). An IC₅₀ value of 70 nM was measured for the 3,4-dichlorophenylalanine containing peptide (P14), which represent a 15, 115 and 4 fold increase as compared to P6, P1 and the full PolIV enzyme, respectively (Table 6 and Table 7). To the contrary, replacement of the terminal phenylalanine by a cyclohexyl-L-alanyl group (Cha), a tryptophan or a 2-amino-tetradecanoic acid (Atda) (P20, P21 and P22, Table 6) led to a decreased affinity.

TABLE 6 Effect of modifications on the  terminal phenylalanine benzyl ring    on the interaction of peptide with   the E. coli β clamp. # sequence IC₅₀ (μM) SEQ ID No: P6 AcQLDLF 1.12 6 P11 Ac-Q Cha DL pMeF 0.26 11 P12 Ac-Q Cha DL pClF 0.16 12 P13 Ac-Q Cha DL pBrF 0.10 13 P14 Ac-Q Cha DL diClF 0.072 14 P20 Ac-Q Cha DL Cha 0.41 20 P21 Ac-Q Cha DLW 0.64 21 P22 Ac-Q Cha DL Atda 3.72 22

The interactions of the various peptides with the β ring were also evaluated using a polymerase competition assay where primer elongation performed by the polIV DNA polymerase is challenged by various concentrations of the peptides to be tested^(17.) While the β independent activity of the polymerase is insensitive to the peptides, indicating that they have no direct effect on the enzyme activity, the β dependent elongation activity is differentially inhibited depending on the affinity of the peptide for its target (FIG. 4). A quantitative analysis of the biochemical assays is shown in FIG. 7. Although this approach is less sensitive than SPR, a good correlation is observed between the two methods (FIG. 4).

TABLE 7 Influence of the C-terminal tripeptide sequence and effect of site specific modifications on  the interaction of peptide with the E. coli β clamp, as measured by SPR experiments. Ki ΔG  Seq se- IC₅₀ (10⁶  (Kcal/ Id # quence (μM) M⁻¹) mol) No: PolIV 0.29 4.7 −9.09 P1 RQLVLGL 8.85 0.15 −7.06  1 P2 RQLVLL 21.53 0.0063 −6.54  2 P3 RQLVLF 8.62 0.15 −7.04  3 P4 RQLVFL 256 Ø Ø  4 P5 QLDLF 12.44 0.11 −6.87  5 P6 AcQLDLF 1.12 1.2 −8.22  6 P7 Ac Q ChaDLF 0.17 8.01 −9.42  7 P8 Ac Q hCha DLF 82.8 16.4 −5.74  8 P9 Ac Q Hol DLF 0.74 1.84 −8.54  9 P10 Ac Q NptGly DLF 0.99 1.36 −8.36 10 P11 Ac-Q Cha DL pMeF 0.26 8.43 −9.44 11 P12 Ac-Q Cha DL pClF 0.16 13.7 −9.73 12 P13 Ac-Q Cha DL pBrF 0.096 13.49 −9.71 13 P14 Ac-Q Cha DL diClF 0.077 17 −9.85 14 Ø: not determined. Ki = (1 + K_(A)[β])/IC50. ΔG = −RT ln Ki. PolIV: E coli DNA polymerase IV. The following table 8 presents the analytical data concerning the most relevant peptides described in this study.

TABLE 8 Sequence and analytical data of C-terminal peptides of the E. coli DNA polymerase IV and analogues (related to FIG. 1 and  table 7). PU- MS  HPLC RITY FOUND SEQ Pep- t_(R) [%] CALC. [M + ID tide compound (min) HPLC MASS H⁺] No: P1 H-Arg-Gln-Leu-Val- 11.97 98.2 799.0 799.9  1 Leu-Gly-Leu-OH— P2 H-Arg-Gln-Leu-Val- 11.50 93.0 740.5 741.9  2 Leu-Leu-OH P3 H-Arg-Gln-Leu-Val- 12.13 92.1 774.5 775.8  3 Leu-Phe-OH P4 H-Arg-Gln-Leu-Val- 12.02 92.5 774.5 775.5  4 Phe-Leu-OH P5 H-Gln-Leu-Asp-Leu- 12.55 90 634.3 634.8  5 Phe-OH P6 Ac-Gln-Leu-Asp-Leu- 13.25 92.5 676.3 677.3  6 Phe-OH P7 Ac-Gln-Cha-Asp-Leu- 15.12 95 716.8 717.7  7 Phe-OH P8 Ac-Gln-hCha-Asp-Leu- 16.23 50.93 730.8 731.4  8 Phe-OH P9 Ac-Gln-Hol-Asp-Leu- 14.23 90 690.8 691.0  9 Phe-OH P10 Ac-Gln-NptGly-Asp- 13.99 94 702.8 691.7 10 Leu-Phe-OH P11 Ac-Gln-Cha-Asp-Leu- 15.88 97 716.4 732.7 11 pMePhe-OH P12 Ac-Gln-Cha-Asp-Leu- 16.27 93 736.3 752.7 12 pClPhe-OH P13 Ac-Gln-Cha-Asp-Leu- 16.45 95 794.3 799.8 13 pBrPhe-OH P14 Ac-Gln-Cha-Asp-Leu- 17.18 91 784.3 786.9 14 (3,4-di-Cl)Phe-OH Cha: beta-cyclohexyl-L-alanyl; hCha: L-homoCha; Hol: L-homoleucyl; NptGly: neopentylglycyl; pMePhe: 4-methyl-L-phenylalanyl; pClPhe: 4-chloro-L-phenylalanyl; pBrPhe: 4-bromo-L-phenylalanyl; (3,4-di-Cl)Phe: 3,4-dichloro-L-phenylalanyl

1.2.4. Crystal Structure of the Modified Peptides β Ring Complexes.

Peptides P12 and P14 were co-crystallized with the β ring. The structures were solved by molecular replacement at 2.6 Å and 1.95 Å resolution respectively, using the 1OK7 structure as a search model. As for the previous P6-β complex, both complexes crystallized in space group P1 but with one ring per asymmetric unit (Table 3). Each monomer of the rings binds a peptide, and both ligands adopt a similar configuration (rmsd=0.70 Å and 0.78 Å for the peptides atoms of P12 and P14, respectively), indicating that the modified peptides essentially adopt the same conformation and location in the binding pocket. The Cha moiety is located at the same position as the L₃ residue of P6 but extends its interaction further within a hydrophobic pocket shaped by the P₃₆₃, V₃₄₄, M₃₆₂ and R₃₆₅ β residues (FIG. 5A) and interacts with the last three residues. These extra interactions probably account for the increased affinity of P7 as compared to P6 (table 1). Additionally, the Nα of this modified residue interacts with the Cα=O of P₃₆₃. The p-chloro and 3,4-dichloro F residues of P12 and P14 are almost superimposed (FIG. 5B) and interact with β residues T₁₇₂, L₁₇₇ and V₂₄₇. The chlorine atom in meta position in P14 establishes an halogen bond with the hydroxyl oxygen of T₁₇₂, with contact distance and angle in good agreement with previously published data (d=3.17 Å and θ=148.71°)²⁰. In contrast, the para chlorine atoms of both P12 and P14 do not establish such type of interaction because the distances with adjacent oxygen atoms of β residues are too large.

The atomic coordinates P12 and P14, co-crystalized with the β clamp, are disclosed in the following Tables 9 and 10. These tables indicate the atomic coordinates of the peptides and of the binding site of the β clamp (residues≦5 Å from the ligand). The other residues have the same positions as in the previously determined structure (PDB 1OK7) also described in U.S. Pat. No. 7,635,583.

TABLE 9 Atomic coordinates of P12 residues and of the residues involved in the binding of P12 to the β clamp, in the crystal of P12 peptide co-crystallized with the β ring. ATOM 1 NH1 ARG A 152 −13.878 13.827 24.977 1.00 41.13 N1+ TER 2 ARG A 152 ATOM 3 CD2 LEU A 155 −9.376 16.306 22.697 1.00 33.78 C TER 4 LEU A 155 ATOM 5 CB THR A 172 −5.579 13.660 24.332 1.00 10.80 C ATOM 6 CG2 THR A 172 −6.162 15.062 24.379 1.00 3.00 C ATOM 7 OG1 THR A 172 −6.396 12.784 25.111 1.00 11.87 O TER 8 THR A 172 ATOM 9 N GLY A 174 −9.375 11.662 24.270 1.00 22.35 N ATOM 10 CA GLY A 174 −10.197 11.573 25.471 1.00 21.94 C ATOM 11 C GLY A 174 −9.904 10.341 26.321 1.00 26.11 C ATOM 12 O GLY A 174 −10.187 10.331 27.522 1.00 24.70 O ATOM 13 N HIS A 175 −9.341 9.286 25.699 1.00 23.67 N ATOM 14 CA HIS A 175 −9.001 8.008 26.347 1.00 23.91 C ATOM 15 C HIS A 175 −7.479 7.869 26.584 1.00 24.57 C ATOM 16 O HIS A 175 −7.046 7.395 27.635 1.00 24.56 O ATOM 17 CB HIS A 175 −9.546 6.822 25.519 1.00 25.86 C ATOM 18 CG HIS A 175 −10.971 7.005 25.080 1.00 30.11 C ATOM 19 CD2 HIS A 175 −11.486 7.140 23.838 1.00 32.30 C ATOM 20 ND1 HIS A 175 −12.005 7.082 25.997 1.00 32.23 N ATOM 21 CE1 HIS A 175 −13.107 7.245 25.288 1.00 31.59 C ATOM 22 NE2 HIS A 175 −12.845 7.286 23.985 1.00 32.09 N ATOM 23 N ARG A 176 −6.685 8.290 25.610 1.00 18.69 N ATOM 24 CA ARG A 176 −5.234 8.264 25.702 1.00 18.59 C ATOM 25 C ARG A 176 −4.604 9.609 25.304 1.00 24.08 C ATOM 26 O ARG A 176 −5.276 10.463 24.713 1.00 23.83 O ATOM 27 N LEU A 177 −3.347 9.832 25.715 1.00 19.54 N ATOM 28 CA LEU A 177 −2.653 11.088 25.503 1.00 18.04 C ATOM 29 CB LEU A 177 −2.873 11.987 26.745 1.00 17.92 C ATOM 30 CG LEU A 177 −1.963 13.211 26.971 1.00 21.84 C ATOM 31 CD1 LEU A 177 −2.328 14.347 26.066 1.00 21.30 C ATOM 32 CD2 LEU A 177 −2.004 13.663 28.416 1.00 22.36 C TER 33 LEU A 177 ATOM 34 CA PRO A 242 −8.284 20.266 27.211 1.00 10.30 C ATOM 35 C PRO A 242 −7.012 20.416 28.076 1.00 16.74 C ATOM 36 CB PRO A 242 −8.798 18.838 27.188 1.00 11.44 C ATOM 37 CG PRO A 242 −8.164 18.255 25.944 1.00 16.16 C ATOM 38 CD PRO A 242 −8.332 19.361 24.955 1.00 11.62 C ATOM 39 N ASP A 243 −7.224 20.463 29.391 1.00 14.24 N ATOM 40 CA ASP A 243 −6.222 20.608 30.438 1.00 14.52 C ATOM 41 C ASP A 243 −5.454 19.282 30.618 1.00 21.86 C ATOM 42 O ASP A 243 −5.842 18.424 31.416 1.00 25.37 O ATOM 43 N TYR A 244 −4.471 19.055 29.748 1.00 15.95 N ATOM 44 CA TYR A 244 −3.688 17.827 29.764 1.00 15.04 C ATOM 45 CB TYR A 244 −2.900 17.645 28.437 1.00 14.90 C TER 46 TYR A 244 ATOM 47 NH1 ARG A 246 −9.401 15.913 36.324 1.00 21.44 N1+ ATOM 48 CB VAL A 247 −4.905 13.902 32.710 1.00 29.59 C ATOM 49 CG1 VAL A 247 −5.480 12.551 33.104 1.00 29.67 C ATOM 50 CG2 VAL A 247 −6.024 14.870 32.343 1.00 29.06 C TER 51 VAL A 247 ATOM 52 O PHE A 278 −11.396 −2.943 23.858 1.00 27.53 O TER 53 PHE A 278 ATOM 54 N ASN A 320 −7.095 −1.216 26.969 1.00 26.00 N ATOM 55 CB ASN A 320 −8.050 −0.275 24.887 1.00 19.44 C ATOM 56 CG ASN A 320 −8.114 −0.289 23.378 1.00 25.87 C ATOM 57 ND2 ASN A 320 −9.197 0.247 22.826 1.00 19.30 N ATOM 58 OD1 ASN A 320 −7.179 −0.699 22.685 1.00 16.42 O TER 59 ASN A 320 ATOM 60 CD2 TYR A 323 −6.890 3.229 24.491 1.00 20.56 C ATOM 61 CE2 TYR A 323 −8.089 3.767 24.028 1.00 21.88 C ATOM 62 cz TYR A 323 −8.254 4.015 22.675 1.00 30.51 C ATOM 63 OH TYR A 323 −9.422 4.548 22.199 1.00 34.44 O TER 64 TYR A 323 ATOM 65 O SER A 343 −8.420 0.538 36.986 1.00 43.35 O ATOM 66 CA VAL A 344 −9.412 2.967 38.022 1.00 37.68 C ATOM 67 C VAL A 344 −8.749 4.286 37.541 1.00 39.37 C ATOM 68 O VAL A 344 −9.430 5.162 37.004 1.00 37.60 O ATOM 69 CB VAL A 344 −10.716 3.216 38.843 1.00 41.47 C ATOM 70 CG1 VAL A 344 −11.660 2.025 38.749 1.00 41.11 C TER 71 VAL A 344 ATOM 72 CB SER A 346 −4.248 7.581 33.672 1.00 36.35 C TER 73 SER A 346 ATOM 74 O VAL A 360 −2.861 8.288 27.992 1.00 21.29 O ATOM 75 CG1 VAL A 360 −2.051 9.515 31.551 1.00 22.57 C TER 76 VAL A 360 ATOM 77 N MET A 362 −5.771 5.847 29.897 1.00 24.01 N ATOM 78 CA MET A 362 −6.824 5.610 30.866 1.00 25.90 C ATOM 79 C MET A 362 −7.277 4.123 30.792 1.00 30.54 C ATOM 80 O MET A 362 −7.461 3.593 29.689 1.00 29.30 O ATOM 81 CB MET A 362 −7.994 6.560 30.587 1.00 29.26 C ATOM 82 CG MET A 362 −8.871 6.790 31.773 1.00 34.65 C ATOM 83 SD MET A 362 −8.107 7.741 33.104 1.00 40.06 s ATOM 84 CE MET A 362 −9.291 7.371 34.427 1.00 36.64 C ATOM 85 N PRO A 363 −7.464 3.423 31.935 1.00 28.36 N ATOM 86 CA PRO A 363 −7.885 2.016 31.872 1.00 27.57 C ATOM 87 C PRO A 363 −9.367 1.784 31.553 1.00 32.79 C ATOM 88 O PRO A 363 −10.161 2.723 31.445 1.00 30.76 O ATOM 89 CB PRO A 363 −7.534 1.508 33.276 1.00 29.22 C ATOM 90 CG PRO A 363 −7.733 2.646 34.141 1.00 33.99 C ATOM 91 CD PRO A 363 −7.288 3.852 33.338 1.00 30.19 C ATOM 92 N MET A 364 −9.735 0.505 31.409 1.00 33.12 N ATOM 93 CA MET A 364 −11.116 0.063 31.233 1.00 34.28 C ATOM 94 C MET A 364 −11.510 −0.892 32.363 1.00 38.91 C ATOM 95 CB MET A 364 −11.392 −0.494 29.827 1.00 37.35 C ATOM 96 CG MET A 364 −10.591 −1.704 29.437 1.00 42.38 C ATOM 97 SD MET A 364 −10.644 −1.982 27.634 1.00 48.21 s ATOM 98 CE MET A 364 −12.355 −2.535 27.415 1.00 45.40 C ATOM 99 N ARG A 365 −12.802 −0.893 32.770 1.00 36.16 N ATOM 100 CA ARG A 365 −13.345 −1.727 33.857 1.00 54.18 C ATOM 101 O ARG A 365 −13.715 −3.627 32.448 1.00 15.77 O ATOM 102 CB ARG A 365 −14.777 −1.280 34.190 1.00 54.40 C ATOM 103 CG ARG A 365 −14.987 −0.808 35.628 1.00 61.81 C ATOM 104 CD ARG A 365 −16.323 −0.093 35.795 1.00 67.50 C ATOM 105 NE ARG A 365 −16.385 0.668 37.050 1.00 70.43 N ATOM 106 CZ ARG A 365 −17.177 1.718 37.255 1.00 69.73 C ATOM 107 NH1 ARG A 365 −17.980 2.151 36.291 1.00 47.59 N1+ ATOM 108 NH2 ARG A 365 −17.162 2.350 38.424 1.00 45.96 N TER 109 ARG A 365 HETATM 110 O HOH A 397 −12.948 −0.572 24.569 1.00 24.99 O HETATM 111 O HOH A 422 −15.206 5.096 38.996 1.00 47.81 O HETATM 112 O HOH A 433 −8.020 1.912 27.730 1.00 9.25 O HETATM 113 O HOH C 24 −11.224 1.716 24.238 1.00 30.48 O HETATM 114 C ACE C 367 −14.628 0.704 29.591 1.00 35.56 C HETATM 115 O ACE C 367 −14.221 1.136 30.667 1.00 30.52 O HETATM 116 CH3 ACE C 367 −15.612 −0.454 29.524 1.00 29.87 C ATOM 117 N GLN C 368 −14.292 1.401 28.359 1.00 30.56 N ATOM 118 CA GLN C 368 −13.424 2.699 28.412 1.00 25.45 C ATOM 119 C GLN C 368 −13.778 3.790 29.559 1.00 32.24 C ATOM 120 O GLN C 368 −14.830 4.442 29.488 1.00 32.94 O ATOM 121 CB GLN C 368 −13.142 3.398 26.993 1.00 29.55 C ATOM 122 CG GLN C 368 −11.862 4.252 26.986 1.00 29.78 C ATOM 123 CD GLN C 368 −10.609 3.491 27.350 1.00 23.65 C ATOM 124 NE2 GLN C 368 −9.792 3.893 28.463 1.00 24.50 N ATOM 125 OE1 GLN C 368 −10.426 2.411 26.809 1.00 27.24 O TER 126 GLN C 368 HETATM 127 N ALC C 369 −12.687 4.147 30.488 1.00 28.64 N HETATM 128 CA ALC C 369 −12.514 5.508 30.964 1.00 29.77 C HETATM 129 C ALC C 369 −11.636 6.300 29.995 1.00 30.06 C HETATM 130 O ALC C 369 −10.844 5.719 29.263 1.00 32.57 O HETATM 131 CB ALC C 369 −11.826 5.516 32.374 1.00 29.41 C HETATM 132 CG ALC C 369 −12.462 4.800 33.632 1.00 27.38 C HETATM 133 CD1 ALC C 369 −11.611 3.659 34.076 1.00 26.63 C HETATM 134 CD2 ALC C 369 −13.892 4.431 33.398 1.00 29.06 C HETATM 135 CE1 ALC C 369 −12.176 2.854 35.208 1.00 26.40 C HETATM 136 CE2 ALC C 369 −14.487 3.799 34.618 1.00 36.93 C HETATM 137 cz ALC C 369 −13.665 2.730 35.276 1.00 30.08 C ATOM 138 N ASP C 370 −11.984 7.629 29.488 1.00 34.94 N ATOM 139 CA ASP C 370 −12.550 8.830 30.301 1.00 34.00 C ATOM 140 C ASP C 370 −11.474 9.729 31.120 1.00 30.52 C ATOM 141 O ASP C 370 −11.377 9.697 32.347 1.00 25.64 O ATOM 142 CB ASP C 370 −14.029 8.546 30.909 1.00 38.94 C ATOM 143 CG ASP C 370 −15.012 8.727 29.752 1.00 40.34 C ATOM 144 OD1 ASP C 370 −16.182 8.114 29.765 1.00 59.35 O ATOM 145 OD2 ASP C 370 −14.418 9.287 28.668 1.00 30.05 O1− ATOM 146 N LEU C 371 −10.560 10.539 30.269 1.00 32.14 N ATOM 147 CA LEU C 371 −9.624 11.571 30.769 1.00 33.35 C ATOM 148 C LEU C 371 −10.269 12.914 31.093 1.00 36.56 C ATOM 149 O LEU C 371 −9.703 13.671 31.882 1.00 38.88 O ATOM 150 CB LEU C 371 −8.465 11.790 29.754 1.00 29.71 C ATOM 151 CG LEU C 371 −7.489 10.663 29.542 1.00 32.92 C ATOM 152 CD1 LEU C 371 −6.748 10.753 28.242 1.00 21.26 C ATOM 153 CD2 LEU C 371 −6.637 10.343 30.752 1.00 23.07 C TER 154 LEU C 371 HETATM 155 N 200 C 372 −11.514 13.352 30.399 1.00 31.07 N HETATM 156 CA 200 C 372 −12.085 14.735 30.622 1.00 32.30 C HETATM 157 C 200 C 372 −13.594 14.731 30.756 1.00 37.62 C HETATM 158 O 200 C 372 −14.200 15.440 31.810 1.00 41.91 O HETATM 159 CB 200 C 372 −11.640 15.817 29.612 1.00 30.34 C HETATM 160 CG 200 C 372 −10.185 15.711 29.135 1.00 25.13 C HETATM 161 CD1 200 C 372 −9.909 15.210 27.863 1.00 20.80 C HETATM 162 CD2 200 C 372 −9.161 16.082 29.995 1.00 22.71 C HETATM 163 CE1 200 C 372 −8.592 15.088 27.455 1.00 22.75 C HETATM 164 CE2 200 C 372 −7.842 15.960 29.593 1.00 22.47 C HETATM 165 CZ 200 C 372 −7.572 15.459 28.324 1.00 26.57 C HETATM 166 CL 200 C 372 −5.931 15.288 27.829 1.00 37.22 CL HETATM 167 OXT 200 C 372 −14.362 13.982 29.828 1.00 44.19 O END

TABLE 10 Atomic coordinates of P14 residues and of the residues involved in the binding of P14 to the β clamp, in the crystal of P14 peptide co-crystallized with the β ring. ATOM 1 NH1 ARG A 152 −13.000 14.676 25.384 1.00 41.87 N1+ TER 2 ARG A 152 ATOM 3 CD2 LEU A 155 −8.771 17.218 22.511 1.00 32.76 C TER 4 LEU A 155 ATOM 5 CB THR A 172 −5.062 14.269 24.018 1.00 29.73 C ATOM 6 CG2 THR A 172 −5.795 15.620 24.122 1.00 26.88 C ATOM 7 OG1 THR A 172 −5.728 13.271 24.786 1.00 28.48 O TER 8 THR A 172 ATOM 9 N GLY A 174 −8.679 12.290 24.030 1.00 26.79 N ATOM 10 CA GLY A 174 −9.451 12.319 25.270 1.00 28.12 C ATOM 11 C GLY A 174 −9.367 11.041 26.069 1.00 32.47 C ATOM 12 O GLY A 174 −9.848 10.979 27.199 1.00 31.98 O ATOM 13 N HIS A 175 −8.715 10.024 25.494 1.00 30.75 N ATOM 14 CA HIS A 175 −8.481 8.734 26.139 1.00 30.80 C ATOM 15 C HIS A 175 −6.976 8.530 26.320 1.00 30.70 C ATOM 16 O HIS A 175 −6.557 8.002 27.335 1.00 29.87 O ATOM 17 CB HIS A 175 −9.131 7.602 25.325 1.00 32.00 C ATOM 18 CG HIS A 175 −10.595 7.832 25.079 1.00 35.24 C ATOM 19 CD2 HIS A 175 −11.259 7.998 23.916 1.00 37.80 C ATOM 20 ND1 HIS A 175 −11.496 7.948 26.130 1.00 37.36 N ATOM 21 CE1 HIS A 175 −12.677 8.158 25.576 1.00 37.24 C ATOM 22 NE2 HIS A 175 −12.587 8.212 24.247 1.00 38.20 N ATOM 23 N ARG A 176 −6.162 8.980 25.337 1.00 25.93 N ATOM 24 CA ARG A 176 −4.709 8.899 25.425 1.00 23.72 C ATOM 25 C ARG A 176 −4.078 10.213 25.009 1.00 27.35 C ATOM 26 O ARG A 176 −4.648 10.950 24.212 1.00 24.69 O ATOM 27 N LEU A 177 −2.885 10.489 25.534 1.00 25.59 N ATOM 28 CA LEU A 177 −2.204 11.738 25.266 1.00 25.33 C ATOM 29 CB LEU A 177 −2.431 12.671 26.488 1.00 25.95 C ATOM 30 CG LEU A 177 −1.853 14.080 26.442 1.00 29.85 C ATOM 31 CD1 LEU A 177 −2.845 15.053 27.040 1.00 31.56 C TER 32 LEU A 177 ATOM 33 CA PRO A 242 −7.701 20.996 27.178 1.00 30.57 C ATOM 34 C PRO A 242 −6.353 21.180 27.870 1.00 35.62 C ATOM 35 O PRO A 242 −5.277 21.201 27.239 1.00 31.98 O ATOM 36 CB PRO A 242 −8.234 19.577 27.285 1.00 32.60 C ATOM 37 CG PRO A 242 −7.827 18.915 26.040 1.00 37.83 C ATOM 38 N ASP A 243 −6.480 21.417 29.176 1.00 34.91 N ATOM 39 CA ASP A 243 −5.451 21.642 30.163 1.00 35.21 C ATOM 40 C ASP A 243 −4.812 20.307 30.500 1.00 38.35 C ATOM 41 O ASP A 243 −5.193 19.613 31.455 1.00 37.41 O ATOM 42 N TYR A 244 −3.865 19.930 29.652 1.00 34.76 N ATOM 43 CA TYR A 244 −3.152 18.662 29.719 1.00 34.71 C ATOM 44 C TYR A 244 −2.245 18.565 30.939 1.00 39.89 C ATOM 45 CB TYR A 244 −2.366 18.444 28.424 1.00 34.64 C TER 46 TYR A 244 ATOM 47 NH1 ARG A 246 −9.362 18.335 34.959 1.00 56.46 N1+ ATOM 48 CB VAL A 247 −4.800 14.658 32.377 1.00 45.61 C ATOM 49 CG1 VAL A 247 −6.175 15.272 32.614 1.00 44.77 C ATOM 50 CG2 VAL A 247 −4.260 15.077 31.013 1.00 45.81 C TER 51 VAL A 247 ATOM 52 O GLY A 318 −7.101 −0.481 31.341 1.00 28.54 O ATOM 53 N PHE A 319 −6.439 −2.005 29.819 1.00 25.76 N ATOM 54 CA PHE A 319 −6.062 −1.039 28.794 1.00 25.29 C ATOM 55 C PHE A 319 −6.489 −1.419 27.399 1.00 26.95 C ATOM 56 O PHE A 319 −6.510 −2.591 27.065 1.00 25.62 O ATOM 57 CB PHE A 319 −4.532 −0.862 28.741 1.00 26.12 C ATOM 58 CG PHE A 319 −3.977 0.004 29.823 1.00 26.52 C ATOM 59 CD1 PHE A 319 −4.229 1.368 29.839 1.00 30.64 C ATOM 60 CE1 PHE A 319 −3.731 2.174 30.867 1.00 31.78 C ATOM 61 N ASN A 320 −6.726 −0.410 26.564 1.00 23.99 N ATOM 62 CA ASN A 320 −6.956 −0.616 25.155 1.00 24.11 C ATOM 63 C ASN A 320 −5.554 −0.926 24.635 1.00 27.35 C ATOM 64 O ASN A 320 −4.654 −0.066 24.695 1.00 24.65 O ATOM 65 CB ASN A 320 −7.569 0.636 24.508 1.00 23.58 C ATOM 66 CG ASN A 320 −7.625 0.595 23.004 1.00 34.43 C ATOM 67 ND2 ASN A 320 −8.716 1.031 22.426 1.00 24.70 N TER 68 ASN A 320 ATOM 69 CB TYR A 323 −3.987 3.146 23.786 1.00 24.01 C ATOM 70 CG TYR A 323 −5.324 3.669 23.316 1.00 27.27 C ATOM 71 CD2 TYR A 323 −6.338 3.953 24.224 1.00 27.85 C ATOM 72 CE2 TYR A 323 −7.551 4.485 23.803 1.00 28.48 C ATOM 73 cz TYR A 323 −7.760 4.733 22.459 1.00 39.89 C TER 74 TYR A 323 ATOM 75 O SER A 343 −8.221 1.305 36.401 1.00 35.47 O ATOM 76 CA VAL A 344 −9.112 3.668 37.518 1.00 31.45 C ATOM 77 C VAL A 344 −8.465 5.023 37.143 1.00 35.04 C ATOM 78 O VAL A 344 −9.164 5.988 36.815 1.00 35.77 O ATOM 79 CB VAL A 344 −10.398 3.839 38.375 1.00 35.78 C ATOM 80 CG1 VAL A 344 −11.101 2.498 38.569 1.00 35.22 C TER 81 VAL A 344 ATOM 82 CB SER A 346 −3.897 8.687 33.543 1.00 33.22 C ATOM 83 OG SER A 346 −3.189 9.493 34.475 1.00 39.59 O TER 84 SER A 346 ATOM 85 C VAL A 360 −1.708 8.441 28.606 1.00 31.20 C ATOM 86 O VAL A 360 −2.317 8.921 27.649 1.00 30.76 O ATOM 87 CB VAL A 360 −0.897 10.498 29.926 1.00 31.93 C ATOM 88 CG1 VAL A 360 −2.031 10.304 30.938 1.00 32.54 C ATOM 89 C VAL A 361 −4.106 6.199 29.791 1.00 28.46 C ATOM 90 CG1 VAL A 361 −3.751 4.254 27.465 1.00 28.23 C ATOM 91 N MET A 362 −5.362 6.501 29.534 1.00 27.43 N ATOM 92 CA MET A 362 −6.410 6.303 30.527 1.00 28.06 C ATOM 93 C MET A 362 −6.834 4.822 30.434 1.00 30.78 C ATOM 94 O MET A 362 −7.011 4.331 29.325 1.00 29.96 O ATOM 95 CB MET A 362 −7.591 7.238 30.220 1.00 30.87 C ATOM 96 CG MET A 362 −8.518 7.477 31.376 1.00 34.33 C ATOM 97 SD MET A 362 −7.736 8.227 32.828 1.00 37.58 s ATOM 98 CE MET A 362 −9.105 8.087 34.005 1.00 34.59 C ATOM 99 N PRO A 363 −6.981 4.091 31.569 1.00 26.83 N ATOM 100 CA PRO A 363 −7.383 2.677 31.502 1.00 25.64 C ATOM 101 C PRO A 363 −8.853 2.456 31.064 1.00 31.65 C ATOM 102 O PRO A 363 −9.600 3.409 30.801 1.00 29.62 O ATOM 103 CB PRO A 363 −7.148 2.203 32.941 1.00 26.32 C ATOM 104 CG PRO A 363 −7.447 3.380 33.754 1.00 30.73 C ATOM 105 CD PRO A 363 −6.797 4.505 32.971 1.00 26.98 C ATOM 106 N MET A 364 −9.251 1.179 30.970 1.00 28.59 N ATOM 107 CA MET A 364 −10.606 0.744 30.632 1.00 28.63 C ATOM 108 C MET A 364 −11.156 0.010 31.848 1.00 33.94 C ATOM 109 O MET A 364 −10.376 −0.602 32.572 1.00 30.49 O ATOM 110 CB MET A 364 −10.559 −0.264 29.473 1.00 30.86 C ATOM 111 CG MET A 364 −10.480 0.379 28.110 1.00 34.27 C ATOM 112 SD MET A 364 −10.177 −0.880 26.856 1.00 37.20 S ATOM 113 CE MET A 364 −11.844 −1.575 26.686 1.00 34.67 C ATOM 114 N ARG A 365 −12.490 0.042 32.084 1.00 36.79 N ATOM 115 CA ARG A 365 −13.039 −0.701 33.225 1.00 38.26 C ATOM 116 C ARG A 365 −13.003 −2.213 32.924 1.00 41.53 C ATOM 117 O ARG A 365 −13.457 −2.647 31.869 1.00 41.37 O ATOM 118 CB ARG A 365 −14.441 −0.209 33.655 1.00 40.76 C ATOM 119 CG ARG A 365 −14.900 −0.881 34.954 1.00 49.70 C ATOM 120 CD ARG A 365 −16.194 −0.321 35.483 1.00 57.33 C ATOM 121 NE ARG A 365 −15.967 0.754 36.451 1.00 62.68 N ATOM 122 CZ ARG A 365 −16.403 1.998 36.291 1.00 83.24 C ATOM 123 NH1 ARG A 365 −17.085 2.335 35.202 1.00 79.01 N1+ ATOM 124 NH2 ARG A 365 −16.159 2.916 37.217 1.00 72.47 N TER 125 ARG A 365 HETATM 126 O HOH A 393 −12.258 0.479 23.904 1.00 30.49 O HETATM 127 O HOH A 434 −11.324 4.559 22.768 1.00 39.46 O HETATM 128 O HOH A 453 −10.378 2.373 23.903 1.00 29.67 O HETATM 129 O HOH A 463 −8.977 20.736 30.414 1.00 61.87 O HETATM 130 O HOH A 490 −11.833 19.233 26.362 1.00 59.30 O HETATM 131 O HOH A 494 −9.709 19.924 32.759 1.00 46.54 O HETATM 132 O HOH A 516 −16.291 −3.291 31.343 1.00 50.13 O HETATM 133 O HOH C 45 −6.988 2.274 27.402 1.00 24.52 O HETATM 134 O HOH C 219 −14.432 8.577 32.316 1.00 42.19 O HETATM 135 O HOH C 236 −12.880 18.057 33.132 1.00 45.93 O HETATM 136 C ACE C 367 −14.148 1.368 29.378 1.00 30.45 C HETATM 137 O ACE C 367 −13.835 1.713 30.502 1.00 36.32 O HETATM 138 CH3 ACE C 367 −15.035 0.134 29.141 1.00 23.39 C ATOM 139 N GLN C 368 −13.746 1.957 28.290 1.00 33.32 N ATOM 140 CA GLN C 368 −12.873 3.100 28.115 1.00 32.69 C ATOM 141 C GLN C 368 −13.283 4.349 28.853 1.00 33.34 C ATOM 142 O GLN C 368 −14.330 4.821 28.622 1.00 32.62 O ATOM 143 CB GLN C 368 −12.617 3.443 26.659 1.00 30.16 C ATOM 144 CG GLN C 368 −11.470 4.502 26.458 1.00 29.48 C ATOM 145 CD GLN C 368 −10.087 4.218 27.096 1.00 29.80 C ATOM 146 NE2 GLN C 368 −9.514 3.231 26.666 1.00 25.11 N ATOM 147 OE1 GLN C 368 −9.495 4.916 28.005 1.00 31.46 O TER 148 GLN C 368 HETATM 149 N ALC C 369 −12.348 4.783 29.710 1.00 32.73 N HETATM 150 CA ALC C 369 −12.500 6.004 30.548 1.00 34.14 C HETATM 151 C ALC C 369 −11.878 7.192 29.752 1.00 36.64 C HETATM 152 O ALC C 369 −10.984 7.069 28.869 1.00 32.46 O HETATM 153 CB ALC C 369 −11.739 5.830 31.889 1.00 34.55 C HETATM 154 CG ALC C 369 −12.265 4.559 32.650 1.00 35.64 C HETATM 155 CD1 ALC C 369 −13.808 4.654 32.962 1.00 39.49 C HETATM 156 CD2 ALC C 369 −11.323 4.206 33.835 1.00 33.24 C HETATM 157 CE1 ALC C 369 −14.214 3.964 34.302 1.00 41.77 C HETATM 158 CE2 ALC C 369 −11.911 3.020 34.514 1.00 36.86 C HETATM 159 CZ ALC C 369 −13.067 3.588 35.295 1.00 40.10 C ATOM 160 N ASP C 370 −12.406 8.334 30.145 1.00 40.84 N ATOM 161 CA ASP C 370 −12.054 9.670 29.629 1.00 43.94 C ATOM 162 C ASP C 370 −10.905 10.238 30.494 1.00 41.92 C ATOM 163 O ASP C 370 −10.900 10.094 31.706 1.00 39.99 O ATOM 164 CB ASP C 370 −13.194 10.609 29.791 1.00 48.03 C ATOM 165 CG ASP C 370 −14.398 10.233 29.003 1.00 55.27 C ATOM 166 OD1 ASP C 370 −14.373 9.336 28.084 1.00 56.63 O ATOM 167 OD2 ASP C 370 −15.398 10.873 29.360 1.00 59.41 O1− ATOM 168 N LEU C 371 −9.929 10.917 29.882 1.00 43.04 N ATOM 169 CA LEU C 371 −8.785 11.787 30.606 1.00 42.42 C ATOM 170 C LEU C 371 −9.335 12.911 31.330 1.00 47.23 C ATOM 171 O LEU C 371 −8.808 13.271 32.377 1.00 47.55 O ATOM 172 CB LEU C 371 −7.621 12.265 29.726 1.00 37.30 C ATOM 173 CG LEU C 371 −6.599 11.206 29.251 1.00 32.23 C ATOM 174 CD1 LEU C 371 −5.841 11.622 27.996 1.00 27.72 C ATOM 175 CD2 LEU C 371 −5.712 10.688 30.370 1.00 32.57 C TER 176 LEU C 371 HETATM 177 N ZCL C 372 −10.331 13.589 30.796 1.00 51.48 N HETATM 178 CA ZCL C 372 −11.007 14.833 31.271 1.00 56.21 C HETATM 179 C ZCL C 372 −12.356 14.343 31.982 1.00 59.74 C HETATM 180 O ZCL C 372 −13.391 14.104 31.274 1.00 58.69 O HETATM 181 CB ZCL C 372 −11.091 16.085 30.288 1.00 55.89 C HETATM 182 CG ZCL C 372 −9.672 16.291 29.799 1.00 57.05 C HETATM 183 CD1 ZCL C 372 −9.347 15.682 28.574 1.00 57.40 C HETATM 184 CD2 ZCL C 372 −8.663 17.020 30.494 1.00 57.44 C HETATM 185 CE1 ZCL C 372 −8.088 15.803 28.089 1.00 59.91 C HETATM 186 CE2 ZCL C 372 −7.347 17.172 29.984 1.00 57.91 C HETATM 187 CZ ZCL C 372 −7.057 16.515 28.746 1.00 57.04 C HETATM 188 CLE1 ZCL C 372 −7.803 15.045 26.635 1.00 66.69 CL HETATM 189 CLZ ZCL C 372 −5.507 16.525 27.966 1.00 50.27 CL HETATM 190 OXT ZCL C 372 −12.339 14.160 33.254 1.00 61.54 O END

1.2.5. Thermodynamic Analysis of the Pseudo Peptides Interaction with the Ring.

ITC experiments were conducted on selected peptides in order to determine the thermodynamics parameters of their interaction with the ring (Table 11 and FIG. 8). Although the K_(d) values determined in these experimental conditions were slightly different from those obtained by SPR, the same tendency was observed for all peptides (Table 11). The largest differences are observed for low affinity peptides (P1, P3, P6) while, for higher affinity peptides (P7, P11, P12, P13 and P14), both techniques yielded similar values. The introduction of modifications, Cha group in position 2 and p-methyl, p-chloro and p-bromo groups on F₅, increases the affinity of the ligand, reaching respectively about 380, 100, 65 and 150 nM range, as compared to the 1-2 μM affinity of the reference natural peptide P1 (Table 11). A strong correlation is observed between ΔH and ΔS values (FIG. 8), which reflects an enthalpy-entropy compensatory effect, already observed in other systems^(21, 22). This correlation accounts for the small ΔG variation among the various peptides (FIG. 8). As reported earlier²³, this correlation suggests that the observed desolvation of the pocket upon ligand interaction plays a major role in the binding process.

TABLE 11 ITC experiments on selected β binding peptides. Beta Pep Kd IC₅₀ ΔH ΔS (cal/ ΔG (μM) (μM) Name N (nM) SPR (nM) (cal/Mol) mol/deg) (Kcal/mol) 30 400 P1 (SEQ ID 1.33 1579 8850 −4087 12.8 −7.9 No: 1) RQLVLGL 1.31 2012 −5500 7.6 −7.7 30 400 P3 (SEQ ID  1.54 2816 8620 −5769 6.05 −7.5 No: 3) RQLVLF 1.45 2320 −5699 6.6 −7.6 30 400 P6 (SEQ ID  1.11 820 1120 −1.13 10⁴ −10.4 −8.1 No: 6) AcQLDLF 1.14 613 −1.11 10⁴ −9.08 −8.3 30 400 P7 (SEQ ID 0.74 246  170 −2.13 10⁴ −41.2 −8.9 No: 7) AcQChaDLF 0.76 222 −2.05 10⁴ −38.5 −8.9 20 400 P11 (SEQ ID 0.95 401  260 −1.48 10⁴ −20.4 −8.6 No: 11) AcQChaDLpMeF 0.95 362 −1.44 10⁴ −19.0 −8.6 20 400 P12 (SEQ ID 1.02 89  160 −1.5  10⁴ −18.1 −9.6 No: 12) AcQChaDLpClF 1.06 122 −1.43 10⁴ −16.3 −9.4 20 400 P13 (SEQ ID 0.85 136   96 −1.81 10⁴ −29.3 −9.3 No: 13 AcQChaDLpBrF 0.83 167 −1.52 10⁴ −20.2 −9.1 20 400 P14 (SEQ ID 0.91 73   77 −1.89 10⁴ −30.8 −9.6 No: 14) AcQChaDLdiClF 0.90 55 −1.81 10⁴ −27.6 −9.8 ΔG = ΔH-TΔS. All experiments were performed at 25° C. Results from SPR experiments (IC50) are added for comparison purposes. N: number of binding sites per β monomer.

The effect of the various modifications introduced in the peptides can be estimated from the ΔΔG values (see Table 12). The Cha moiety in position 2 contributes to the increased interaction by −0.66 kcal/mol as compared to the natural pentapeptide P6. While the introduction of a methyl substitution on the ring of the terminal phenylalanine residue is detrimental to the affinity (+0.25 kcal/mol), halogen modification results in a graduate effect on ligand affinity (p-bromo<p-chloro<3,4-dichloro). The chloro modification in para position contributes for the main part to the increased affinity (−0.6 kcal/mol). This contribution can result partly from an increased hydrophobic character of the halogen modified F residue²⁴, as well as from dehydration of both peptide and sub site 1²⁵ and reorganization of water molecules as observed by comparing free and bound β interacting pockets (FIG. 1AB). In comparison, the second chlorine atom (meta position) in P14 only contributes for −0.2 kcal/mol and the para-bromo modification contribution to the binding energy is two times lower (−0.28 kcal/mol) than the corresponding para-chloro modification.

TABLE 12 effects of modifications introduced in the various pentapeptides measured from the ΔΔG values extracted from ITC experiments data. The P6 sequence (AcQLDLF) is chosen as a reference. ΔΔG = ΔGj − ΔGi. ΔGi/ΔGj P6/ P7/ P7/ P7/ P7/ P11/ P11/ P11/ P12/ P13/ P13/ P7 P11 P12 P14 P13 P12 P14 P13 P14 P12 P14 Compared L/ F/ F/ F/ F: pMeF/ pMeF/diClF pMeF/ pClF/ pBrF/ pBrF/ residues Cha pMeF pClF diClF pBrF pClF pBrF diClF pClF diClF ΔΔG −0.66 +0.25 −0.6 −0.8 −0.28 −0.85 −1.05 −0.53 −0.2 −0.32 −0.52 (Kcal/mol)

1.3. Discussion.

1.3.1. The Fully Efficient β Binding Pocket is Formed Upon Ligand Binding.

Many cellular factors involved in replication and genome integrity survey processes require a peptide mediated interaction with the replisome sliding clamp in order to fulfill their function. This interaction has been structurally fully characterized in prokaryotes^(16, 17, 18) and eucaryotes² ²⁶. In our previously published structure (1OK7), the β ring interacts with only one peptide, leaving one binding pocket free¹⁷. This gives the opportunity to compare the structure of a peptide-free versus a peptide-bound pocket and to get insights into the dynamic of the pocket upon peptide binding. Although the general structures of the free or bound pockets are similar, as estimated by the Cα chain conformation (rmsd=0.36 Å), the side chains of several residues undergo major movements (FIG. 6), notably residues M₃₆₂ and S₃₄₆ side chains which are displaced in a concerted way, and residue R₃₆₅. The concerted shift of M₃₆₂ and S₃₄₆ side chains probably is a structural marker for the presence of a peptide ligand in the binding pocket. In absence of the ligand, these residues adopt a so-called close conformation where the M₃₆₂ side chain is oriented toward residue H₁₇₅ and separates subsite 1 and subsite 2 (PDB IDs 2POL, 1MMI and 1OK7 (monomer A)) (FIG. 6). Alternatively, in all β ring bound structures (1UNN, 1OK7 (monomer B), 3D1E, 3D1F), the residues are shifted by 180° in an open conformation, allowing the opening of a cleft joining the two subsites (FIG. 1BC). One exception is found in the structure of β co-crystallized with the Pol II peptide (3D1E) where no ligand is observed in monomer B, although the two residues adopt an open conformation¹⁸. This may result from a partial occupancy of the pocket, making difficult the detection of the peptide.

Residue R₃₆₅ is also shifted by an angle of 46° toward residue L₃₆₆, triggering the opening of a platform shaped by R₃₆₅, P₃₆₃, M₃₆₂ and V₃₄₄, where the L₃ residue of the peptide locates (FIG. 1BC). The global dynamic of this structural modification has been modeled, showing that, as the ligand binds into the pocket, a groove forms that joins subsite 1 and subsite 2, in which the extended peptide can adapt.

The correlation between side chains orientations and the presence of a peptide in the binding pocket suggests that these two side chains might play a strategic function in the ligand binding process. The pocket could adopt two configurations: a closed configuration where the M₃₆₂ side chains lies in the path between the two subsites of the pocket, thus impeding the formation of an efficient binding site (FIG. 1A). Alternatively, an open configuration where the M₃₆₂ side chains shifts by about 180°, allowing ligand binding into the groove that joins the two subsites, as well as the opening of the platform so that the peptide establishes optimal interactions. At present, it is not known if the fully efficient binding pocket is readily available at the surface of the protein, or is structured by the binding of a specific ligand, according to a bona fide induced fit model. Previous observations for a ligand binding site on the cytokine IL-2²⁷ reveal that a portion of the binding site is adaptive and can form a hydrophobic channel upon ligand binding. A similar adaptive process could occur for the β binding pocket and would ensure the binding specificity of ligand proteins. Although the details of the dynamic process of this binding site formation are not known yet, this observation is likely to have major implications for the design of high affinity ligands.

1.3.2. Structure-Based Design of High Affinity Ligands.

The inventors have studied the interaction of various peptides with the E. coli processivity ring. Starting from the natural sequence of the Pol IV DNA polymerase interacting peptide (P1, RQLVLGL, SEQ ID No: 1), they have sequentially modified the peptide in order to increase its interaction strength and concomitantly to limit its size. The affinities were measured by biochemical assays, SPR and ITC, and despite a difference in sensitivity of these various techniques, the relative affinities of the peptides were similar. From the P1-β complex (PDB 1OK7), a minimal acetylated peptide (P6, AcQLDLF) was first delineated and found to bind about eight times tighter than P1. Then, the crystallographic structure of the P1-β and P6-β complexes were used together with modelling informations to guide modifications at specific positions (corresponding to residues L₂ and F₅ of P6, AcQLDLF), resulting in a dramatic increase of the modified peptides affinity for the β ring interacting pocket. This gain essentially results from an increase in hydrophobic interactions. A first improvement was achieved by the introduction of a cyclohexylalanyl residue at the P6-L2 position. Although position 2 does not correspond to a major binding pocket on the β-clamp, the simple modification to a cyclohexylalanyl residue (P7) improved the affinity by a factor 6 with respect to the P6 peptide (Table 1), indicating that shallow secondary sites can still prove useful in a global optimization scheme.

The other interesting gain was achieved by the addition of halogen atoms on the benzyl ring of the F residue. Interestingly, the chlorine atom when introduced at the meta position, forms a halogen bond that further strengthens the interaction. P14, which combines both the Cha and a 3,4-dichloro phenylalanine residue, displays the highest affinity, around 60 (±10) nM as measured by ITC, which represents a 10 to 15 fold increase in binding as compared to P6 and a 4 fold increased as compared to the whole PolIV DNA polymerase. The double F ring substitution contributes for −0.8 kcal/mol to the overall interaction. The para-chloro substitution provides most of the contribution to the binding while the meta-chloro, involved in a halogen bond, only contributes for one fourth of the total free energy.

Interestingly, halogen substituted aromatic ligands were also recently independently identified for the β-clamp using a chemical library screening strategy. A compound, named RU67, was identified as a β ligand inhibiting the E. coli pol III enzyme with a Ki of 10 μM¹⁸. It contains a di-bromo substituted aromatic ring which is deeply inserted in sub site 1 and almost superimposes with the peptide L₄ residue of the Pol IV binding peptide. One bromine atom does not form any interaction while the other forms a halogen bond with β residue T₁₇₂ (d=3.02 Å and θ=133.21°). This latter interaction is similar to that observed for the chlorine atom in meta position of F₅ in P14.

This specific positioning of halogen substituted ligands in subsite 1 of the β-clamp contrasts with the wider range of positions occupied by natural amino acids in the same pocket. Indeed, the comparison of different structures of β in complex with various natural peptides (1OK7, 3D1E, 3D1F)^(17, 18) reveals that, while most of the peptide residues adopt the same overall conformation within the pocket, the position of the last C-terminal residues of the interacting sequence varies in subsite 1. This suggests that no specific interaction is established with specific β residue, but instead that this hydrophobic interaction, delocalized in all subsite 1, contributes mainly to paste the peptide onto the β surface.

In conclusion, the structure-based approach described herein allows the design of ligands that bind two orders of magnitude tighter than the natural peptide P1, reaching the 10⁻⁸ M range, and 4 fold more than the Pol IV enzyme. This increase in affinity relies both on chemical substitutions introduced on the peptide that increase the hydrophobic interactions and on the fact that the bidentate interaction of the ligand in the binding pocket is retained. This interaction mode promotes the modeling of an efficient binding site, possibly through an induced-fit process.

Further designs of high affinity ligands should also take into account the dynamic nature of the binding site formation. These observations are likely to have major implications for the development of new antibiotic compounds.

Example 2 Synthesis and Functional Characterization of Further Peptides Derived from P7 (SEQ ID No: 7), P12 (SEQ ID No: 12), P14 (SEQ ID No: 14), P23-P30 (SEQ ID Nos: 23-30)

Compounds P23 to P28 (SEQ ID No: 23 to SEQ ID No: 28), which are analogues of P7 and P14, have been prepared to (i) remove carboxylic side functions and monitor the influence of R and guanidino groups and conformational constraint. Compounds P23 to P28 have been prepared as previously reported in example 1 starting from Fmoc-Phe Wang resin or from o-chlorotrityl chloride resin. The linear precursor of P29 (Ac-RQChaKLF-OH, P27, SEQ ID No: 27) is prepared as described in example 1. In the case of compound P30 (SEQ ID No: 30), the linear precursor P28 (SEQ ID No: 28) requires the use of a beta amino acid (Fmoc-beta-HPhe-OH) which is commercially available from Sigma-Aldrich. In the case of P29 and P30, lactam formation between the amino group of the lysine side chain and the C-terminal carboxylic function was performed using standard methods as described in the literature (Robert C. Reid, Giovanni Abbenante, Stephen M. Taylor, and David P. Fairlie; J. Org. Chem., 2003, 68 (11), pp 4464-4471). The cyclization which requires the activation and coupling of a peptidyl-Arg residue (P27 or P28) was carried out in DMF at room temperature using BOP as a coupling agent and diisopropylethylamine as the base and monitored by analytical

RP-HPLC. Briefly, a solution of the fully deprotected peptide 27 or 28 (1 equivalent) in DMF (10-1M) and diisopropylethylamine (5.5 equivalent) was stirred at room temperature (about 20° C.) until homogeneous. Benzotriazole-1-yl-oxy-tris-(dimethylamino)-phosphonium hexafluorophosphate or BOP reagent (1.08 equivalent) was added, and the solution was stirred at room temperature (about 20° C.) for 2 h. A saturated Bicarbonate solution was added and the solid product was filtered off, washed on the filter with ether, and dried under high vacuum. The crude product was dissolved in 50% CH₃CN/50% water (1 L) and purified by C18 RP-HPLC. The peak containing the cyclic peptide was collected and lyophilized.

Example 3 New Peptides Derived from P14, SPR Experiments

Based on the crystals analysis, the inventors have used peptide P14 (Bu29) as a scaffold to derive new variants with the aim of deciphering new properties to the ligand such as increased affinity, increased solubility, or increased cell penetration.

TABLE 13 sequences of the P14 derivative peptides. Peptide name Sequence Mw SEQ ID No: P14 Ac Q Cha D L diClF 14 P14-4 Ac R Q Cha N L diClF 940 36 P14-5 Cin R Q Cha N L diClF 1028 37 P14-6 Ac R Q Cha R L diClF 980.5 38 P14-7 Ac R Q Cha R L F 913 39 P14-8 Ac R Q Cha K L F 886 40 Ac: acetyl group; Cin: cinnamoyl group.

The rationale directing the design of these peptides was 1) to extend the interaction of the N-terminal part of the peptide with the edge of the binding pocket, 2) to take advantage of the position of the D residue of peptide P14, that points towards the solvent, to introduce positively charged residues that will increase the solubility and the membrane penetrating capabilities of the peptide.

The affinity of these peptides for the binding pocket was measured by SPR, by defining the concentration of ligand that challenges 50% of the binding of the natural peptide P1 (RQLVLGL), thus defining a Ki. Results are given in table 14

TABLE 14 relative affinity of P14 derivative pep-  tides for the beta ring binding pocket,  as measured by SPR experiments. Peptide Ki SEQ ID name Sequence (nM) No: P7 Ac Q Cha D L F 170  7 P14 Ac Q Cha D L diClF 85 14 P14-4 Ac R Q Cha N L diClF insoluble 36 P14-5 Cin R Q Cha N L diClF 3250 37 P14-6 Ac R Q Cha R L diClF >10 000 38 P14-7 Ac R Q Cha R L F >10 000 39 P14-8 Ac R Q Cha K L F 904 40

CONCLUSIONS

None of the peptides is a better binder than P14. One of them, P14-8, still bind the target with a 1 μM affinity.

As compared to P14 (SEQ ID No: 14), the introduction of R and K residues in the sequence (P14-8, SEQ ID No: 40) results in a 10 fold decrease in affinity.

The diClF to F substitution results in a 2 fold decrease in affinity (compare P14 and P7). Thus the R and K subtitutions in peptide P14-8 may contribute to a 5 fold decrease (1000/200) of the affinity as compared to P14.

P14-7 and P14-8 differ from each other by the 5^(th) residue (R for #7 and K for #8). Comparison of the relative affinity of these two peptides suggests that K residue is a better choice to maintain affinity. Moreover, comparing P7 and P14-7, which differ by R1 and R5 residues, indicates an affinity ratio of at least 50 (#10 000/200).

Altogether, this suggests that the K5 residue in P14-8 may contribute to decrease the affinity by a factor of 10 (P14-7/P7=5 and P14-8/P7=50).

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The invention claimed is:
 1. A compound of formula (I)

wherein Gln is glutamine; R is selected from the group consisting of a C₁₋₁₂-alkyl group optionally substituted by a C₆₋₁₀-aryl group, a C₂₋₁₂-alkenyl group optionally substituted by a C₆₋₁₀-aryl group, a C₃₋₆-cycloalkyl group, a C₆₋₁₀-aryl group optionally substituted by a C₁₋₄-alkyl, and a C₁₋₅-alkyl-(O—CH₂—CH₂)_(t)— group with t being an integer from 0 to 20 inclusive; R¹ is the side chain of arginine or lysine; R² is a —(CH₂)—C₃₋₆-cycloalkyl group optionally substituted by a halogen and/or by a group selected from the group consisting of —NH₂, —NH—CO—R^(a), —CO₂H, —NHR^(a) and —NR^(a)R^(b), wherein R^(a) and R^(b) are independently a C₁₋₄-alkyl group; R³ is selected from the group consisting of a C₁₋₈-alkyl group, the side chain of arginine or lysine, —(CH₂)_(q)—CO₂R^(7a), —(CH₂)_(q)—CO—NHR^(7b), —CH₂OR⁸ and —(CH₂)_(q)NHR⁹, wherein q is 1, 2, 3 or 4, R^(7a) is a hydrogen atom, a C₁₋₈-alkyl group, a C₄₋₁₂-alkylene group forming together with R⁶ a lactone or a polyether ring, or a C₄₋₁₂-alkenylene, forming together with R⁶ a lactone or a polyether ring, R^(7b) is a hydrogen atom, a C₁₋₈-alkyl group, or —(CH₂)_(q′)—NH— with q′ being an integer between 2 and 8 inclusive and forming together with R⁶ a lactam, R⁸ is a hydrogen atom, a C₁₋₈-alkyl group, a C₄₋₁₂-alkylene group forming together with R⁶ a lactone or a polyether ring, or a C₄₋₁₂-alkenylene, forming together with R⁶ a lactone or a polyether ring, R⁹ is a hydrogen atom, or R⁹ together with R⁶ form a lactam; R⁴ is a C₁₋₈-alkyl group optionally substituted by a C₃₋₆-cycloalkyl group, or a halogen-C₁₋₄-alkyl group; R⁵ is selected from the group consisting of a —(CH₂)—C₃₋₆-cycloalkyl group; —(CH₂—CH₂)—C₃₋₆-cycloalkyl group; a —(CH₂)—C₆₋₁₀-aryl group optionally substituted by a halogen, a C₁₋₂ alkyl group and/or a C₁₋₂ alkoxy group; a —(CH₂—CH₂)—C₆₋₁₀-aryl group optionally substituted by a halogen, a C₁₋₂ alkyl group and/or a C₁₋₂ alkoxy group; a —(CH₂)—C₆₋₁₀-heteroaryl group optionally substituted by a halogen and/or a C₁₋₂ alkyl group; and a —(CH₂—CH₂)—C₅₋₁₀-heteroaryl group optionally substituted by a halogen and/or a C₁₋₂ alkyl group; R⁶ is —CO₂H, —CO₂R¹⁰, —CO—NH₂, —CO—NHR¹⁰, —OR¹⁰ when r is 1 or 2, —NH—CO—NHR¹⁰ when r is 1 or 2, or R⁶ is —CO—, —CO—O— or —O— and forms a lactam, a lactone, or a polyether ring with R^(7a), R^(7b), R⁸ or R⁹; wherein R¹⁰ is a C₁₋₈-alkyl group optionally substituted by a C₆₋₁₀-aryl group; a C₃₋₆-cycloalkyl group; a C₆₋₁₀-aryl group optionally substituted by a halogen, a C₁₋₂-alkyl group and/or a C₁₋₂-alkoxy group; m is 0 or 1; n is an integer from 0 to 9 inclusive; p is an integer from 0 to 10 inclusive; r is 0, 1 or
 2. 2. The compound of claim 1, wherein R is a C₁₋₈-alkyl group optionally substituted by a C₆₋₁₀-aryl group, a C₂₋₈-alkenyl group optionally substituted by a C₆₋₁₀-aryl group or a C₁₋₆-alkyl-(O—CH₂—CH₂)_(t)— group with t being an integer from 0 to 10 inclusive.
 3. The compound of claim 1, wherein R is a C₁₋₄-alkyl group optionally substituted by a C₆₋₁₀-aryl group or a C₂₋₄-alkenyl group optionally substituted by a C₆₋₁₀-aryl group.
 4. The compound of claim 1, wherein n is an integer comprised between 1 and 5 inclusive.
 5. The compound of claim 1, wherein n is
 0. 6. The compound of claim 1, wherein R² is a —(CH₂)—C₃₋₆-cycloalkyl group.
 7. The compound of claim 1, wherein R⁴ is a C₁₋₅-alkyl group or a C₁₋₂-alkyl group optionally substituted by a C₃₋₆-cycloalkyl group.
 8. The compound of claim 1, wherein R⁵ is a —(CH₂)—C₆₋₁₀-aryl group optionally substituted by a halogen, a C₁₋₂ alkyl group and/or a C₁₋₂ alkoxy group.
 9. The compound of claim 1, wherein R³ is selected from the group consisting of a C₁₋₈-alkyl group, the side chain of arginine or lysine, —(CH₂)_(q)—CO₂R^(7a), —(CH₂)_(q)—CO—NHR^(7b), —CH₂OR⁸, and —(CH₂)_(q)NHR⁹, wherein q is 1, 2, 3, 4, R^(7a) is a hydrogen atom, or a C₁₋₈-alkyl group, R^(7b) is a hydrogen atom, or a C₁₋₈-alkyl group, R⁸ is a hydrogen atom, a C₁₋₈-alkyl group, R⁹ is a hydrogen atom; and R⁶ is —CO₂H, —CO₂R¹⁰, —CO—NH₂, —CO—NHR¹⁰, —OR¹⁰ when r is 1 or 2, —NH—CO—NHR¹⁰ when r is 1 or 2; wherein R¹⁰ is a C₁₋₈-alkyl group optionally substituted by a C₆₋₁₀-aryl group; a C₃₋₆-cycloalkyl group; a C₆₋₁₀-aryl group optionally substituted by a halogen, a C₁₋₂-alkyl group and/or a C₁₋₂-alkoxy group.
 10. The compound of claim 1, wherein R³ is selected from the group consisting of the side chain of arginine, the side chain of lysine, —(CH₂)_(q)—CO₂R^(7a) and —(CH₂)_(q)—CO—NHR^(7b), wherein q is 1, 2, 3 or 4, R^(7a) is a hydrogen atom, or a C₁₋₈-alkyl group, and R^(7b) is a hydrogen atom, or a C₁₋₈-alkyl group.
 11. The compound of claim 1, wherein R⁶ is —CO₂H or —CO—NH₂.
 12. The compound of claim 1, wherein R³ is selected from the group consisting of —(CH₂)_(q)—CO₂R^(7a), —(CH₂)_(q)—CO—NHR^(7b), —CH₂OR⁸, and —(CH₂)_(q)NHR⁹, wherein q is 1, 2, 3 or 4, R^(7a) is a C₄₋₈-alkylene group forming together with R⁶ a lactone or a polyether ring, or a C₄₋₈-alkenylene, forming together with R⁶ a lactone or a polyether ring, R^(7b) is —(CH₂)_(q′)—NH— with q′ being an integer from 2 to 8 inclusive and forming together with R⁶ a lactam, R⁸ is a C₄₋₈-alkylene group forming together with R⁶ a lactone or a polyether ring, or a C₄₋₈-alkenylene, forming together with R⁶ a lactone or a polyether ring, R⁹ together with R⁶ form a lactam; R⁶ is —CO—, —CO—O— or —O— and forms a lactam, a lactone, or a polyether ring with R^(7a), R^(7b), R⁸ or R⁹.
 13. The compound of claim 1, which is selected from the group consisting of Ac-Gln-Cha-Asp-Leu-Phe (SEQ ID NO:7), Ac-Gln-Cha-Asp-Leu-pMePhe (SEQ ID NO:11), Ac-Gln-Cha-Asp-Leu-pClPhe (SEQ ID NO:12), Ac-Gln-Cha-Asp-Leu-pBrPhe (SEQ ID NO:13), Ac-Gln-Cha-Asp-Leu-diClPhe (SEQ ID NO:14), Ac-Gln-Cha-Asp-hLeu-Phe (SEQ ID NO:16), and Ac-Gln-Cha-Asp-Cha-Phe (SEQ ID NO:17).
 14. The compound of claim 1, wherein the affinity of said compound for the interacting pocket of the bacterial β ring is at least twice the affinity of the acetylated peptide of sequence AcQLDLF (SEQ ID NO:6) with said interacting pocket.
 15. A pharmaceutical composition comprising, as an active agent, a compound according to claim
 1. 16. A method of treating an infection by E. coli in a subject in need thereof comprising administering the compound of claim 1 to the subject. 